Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe₂ Crystal with Time‐Resolved Terahertz Spectroscopy

Abstract

are new types of semiconductors in which adjacent layers are held by van der Waals force. [1] TMDs have many rich physical properties such as extremely weak phonon-assisted photoluminescence (PL) in bulk structures, relatively strong PL yield in monolayer structures, [2][3][4][5] strong valley-dependent absorption in visible range, [6][7][8] and strong nonlinear optical response. [9][10][11][12] By taking advantage of these properties, significant progresses have been made in the development of electronic and optoelectronic technologies by these materials, [13,14] such as photodetectors, [15] field-effect transistors, [16][17][18] solar cells, [19,20] light-emitting diodes, [21,22] valleytronics, [8,23] integrated circuits, [24] and so on.The exciton binding energy in the monolayer TMDs has been observed in the range of 0.1-1.1 eV, [25][26][27][28] which is larger than that in traditional bulk semiconductors and traditional quantum well. [29,30] This rich excitonic state makes it possible to observe the complex exciton dynamics at room temperature, [31] such as charged and neutral phase of carriers, [32] trions, [33] and biexciton. [34] Meanwhile, bounded exciton could not only dominate the optical response but also play an important role in the optoelectronic processes, such as photocurrent generation and photoconduction in TMD semiconductors. [33,35,36] As a typical TMD material, the bulk WSe 2 has an indirect bandgap of 1.2 eV, and it would transform to the ≈1.65 eV direct bandgap as decreasing to monolayer. [37,38] WSe 2 has been widely used in optoelectronic devices due to its high absorption coefficients in the visible range. Field-effect transistors based on single crystal WSe 2 have been achieved and a carrier mobility comparable to silicon at room temperature was demonstrated. [39] Besides, WSe 2 can also play an important role in the development of van der Waals heterostructures, [40][41][42] in which long-lived interlayer excitons have been observed.Recently, ultrafast time-resolved photoexcited carrier dynamics has been studied in layered TMDs by the most widely used tools, such as optical-pump optical-probe spectroscopy, [43] PL spectroscopy, [38] photocurrent spectroscopy, [36] and electroluminescence. [35,44] The prevalentThe dynamics of photoexcited species is quite important for the development of next-generation ultrafast optoelectronic devices based on transition metal dichalcogenides (TMDs). Herein, time-resolved optical pump terahertz (THz) probe spectroscopy, which is sensitive to both bounded excitons and free electrons/holes, is employed to study the dynamics of photo-induced carriers in the typical layered TMDs crystal tungsten diselenide (WSe 2 ). Initial photoexcitation could generate both free carriers and excitons. The free carriers decay followed by phonon-assistance (≈30 ps) and defect-assistance (≈200-280 ps). The excitons decay followed by the phonon-assisted recombination (≈100 ps) and the defect-induced exciton separation (≈40-200 ps). With the increasing of pump fluence, more f...

Excitons in 2D perovskites for ultrafast terahertz photonic devices

“…We used the OPTP to unveil the dynamics of free carriers and excitons in the 2D perovskite thin film. In OPTP experiment, the attenuation in peak amplitude of THz pulse is proportional to the real part of the photoconductivity, which mostly arises due to in-phase motion of free electrons with the THz electric fields (45)(46)(47)(48). The dynamics of THz amplitude change is recorded by varying the pump delay time scanning at the peak of the THz pulse as shown in Fig.…”

“…1E. While on the other hand, the photoinduced phase shift of the THz pulse is proportional to the imaginary part of the photoconductivity, which arises because of the out-of-phase motion of free carriers with THz electric field and the polarizability of excitons (45)(46)(47)(48). The dynamics of the THz phase is mapped out by pump delay time scanning at the zero crossing of the THz pulse, as shown in Fig.…”

“…Following the previous studies on transition metal dichalcogenides (47,51), to unveil the exciton dynamics in the 2D perovskite, we fitted the measured imaginary part of photoconductivity ( Fig. 2B) using biexponential decay function and the fitted parameters are shown in Table 2.…”

Terahertz Spectroscopy of Nanomaterials: a Close Look at Charge‐Carrier Transport

“…Herein is presented a review of the experiments performed on nanostructures of conventional semiconductors and on carbon nanomaterials (graphene and carbon nanotubes) during the past decade. [13][14][15] Large organic molecules and polymer structures may be also regarded as assemblies of nanoelements, i.e., polymer chains [16] (bottom-left panel of Figure 1). the advantages of small size and efficient charge transport along the axial direction.…”

Time‐Resolved Terahertz Spectroscopy Studies on 2D Van der Waals Materials

“…[117] The measured time-resolved real and imaginary parts of the complex photoconductivities pumped with different power intensities are shown in Figure 11c. [117] The measured time-resolved real and imaginary parts of the complex photoconductivities pumped with different power intensities are shown in Figure 11c.…”