| Recently,two-dimensional(2D)transition metal dichalcogenides(TMDs)materials and their van der Waals(vdW)heterostructures,having extraordinary photoelectric properties and rich physical effects,have attracted extensive attentions.2D-TMDs feature the large-range tunability in bandgap,high carrier mobility,high quantum efficiency,and fast photoelectric response,etc.,which makes them promising as the prototypical materials supporting the development of new generation of micro-electric and photoelectric devices.Benefiting from the atomic thickness in 2D-TMDs,the Coulomb screening effect is weaken,and the quantum confinement effect is significantly enhanced.As a result,many interesting physical phenomena,such as exciton,valley exciton,and many body effect have emerged in 2D-TMDs.In particular,the vdW heterostructures with different kinds of band offset can be constructed by combining diverse 2D-TMDs,which can break through the limitations of lattice match in traditional heterostructures.The heterostructures not only are able to greatly enrich the functions of individual 2D-TMDs,but also can host new physical paradigms,such as interlayer excitons,moire excitons,etc.In 2D materials,the carrier dynamics processes including hot electron cooling,exciton formation,charge transfer,coherent phonon,interband recombination,and others,commonly occur over the timescale ranging from subpicoseconds to nanoseconds.Deeply understanding of carrier dynamics can provide key information and important insights for the photoelectric physics of 2D-TMDs,which is vital for the development of the devices.In this thesis,we thoroughly studied the carrier dynamics of the 2D-TMDs and corresponding heterostructures,represented by the PtS2,ReSe2,ReSe2/MoS2,etc.,mainly using the time resolved ultrafast spectroscopy in the near-infrared and terahertz(THz)spectra range.We have achieved some significant breakthroughs and new insights in the ultrafast photophysics in 2D-TMDs including the formation path of interlayer excitons,the correlations between the hot electron relaxation and electronic structures,the excitation of coherent phonons,etc.,which can provide valuable guidelines and benchmarks for the development of optoelectronic devices based on 2D PtX2,ReX2,MoS2/ReX2(X=S,Se)and their heterostructures.The main research contents and achievements are as follows:(1)We studied the hot electron dynamics in 2D PtS2 and PtSe2 and observed the ultrafast phenomena such as layer dependent saturable absorption(SA)and coherent phonon excitation,which can be explained in terms of calculated electron and phonon structure:In PtS2,we performed the optical pump-probe spectroscopy(OPPS)to study the 1L-4L PtS2 nanoflakes.We observed that the SA effect was greatly reduced and the carrier relaxation time was shortened pronouncedly,with the increase of layer numbers.In addition,the coherent phonons oscillations originating from interlayer shear modes were also detected,showing strong anharmonicity.Moreover,we carried out the time resolved THz spectroscopy(TRTS)to access the transport dynamics in PtSe2 thin films.We found that the layer numbers can effectively modulate the ratio between the excitons and the free carriers in non-equilibrium carriers dynamics,which is closely related to the semiconductor-semimetal phase transition in PtSe2.(2)We interrogated the hot electron dynamics in monolayer ReSe2 nanoflake and film,in which we identified the excitons formation process,and found the nonlinear absorption dynamics caused by the hot electrons have significant in-plane anisotropy:First,using polarization-resolved OPPS,we revealed that the relaxation dynamics of monolayer ReSe2 consists of nonlinear SA and absorption enhancement were dominated by the hot electrons and excitons,featuring the subpicosecond and 35 picosecond time scales,respectively.Interestingly,these carrier dynamics are highly dependent on the laser polarization.The typical anisotropic ultrafast dynamics can be reasonably interpreted based on the dispersion relation of energy band structure and the non-uniform distribution of hot electrons in momentum space.Furthermore,the dynamics of exciton formation at subpicosecond scale are distinguished by analyzing the transient THz photoconductivity spectra.(3)We investigated the ultrafast dynamics in monolayer MoS2/ReSe2 heterostructure,where we identified the charge transfer,free carrier evolution and interlayer exciton relaxation pathways and intermediate processes ranging from subpicoseconds to hundreds of picoseconds timescales:First,the bursts of the ultrafast interfacial current stemming from charge transfer along the stacking direction of the heterostructure are observed using the THz emission spectroscopy with a timescale about 170 fs.Furthermore,using the TRTS and the near-infrared OPPS,we explicitly unveiled an intermediate free-carrier relaxation pathway,and the long-lived dynamics(365 ps)originating from the interlayer excitons recombination,which is more than 10 times longer than that of isolate ReSe2 layer.(4)We studied the correlations between ultrafast dynamics and crystal orientation,thickness in 2D PdSe2 nanoflakes,and found that the hot electron relaxation and coherent phonon excitation are strongly dependent on the sample thickness and crystalline orientation:The PdSe2 nanoflakes with the thickness of 2.35 nm,6.68 nm,16.37 nm and the bulk counterpart have been studied using polarization-resolved OPPS.It is found that the hot electron relaxation dynamics depend on the relative angle between laser polarization and crystal orientation,and the anisotropy effect increases with the increase of thickness.For thick PdSe2,the carrier relaxation lifetime increases by up to 44%due to the change of crystal orientation,showing unusual anisotropic hot electron behavior.Combining with theoretical analysis,we proposed that the in-plane anisotropy in photocarrier dynamics had close correspondences with the non-uniform k space distribution of electron effective mass basing on the energy bands calculations. |
