Role of Photoinduced Exciton in the Transient Terahertz Conductivity

Article pubs.acs.org/JPCC

Role of Photoinduced Exciton in the Transient Terahertz Conductivity of Few-Layer WS2 Laminate Xiao Xing,† Litao Zhao,‡ Zeyu Zhang,† Xiankuan Liu,† Kailin Zhang,† Yang Yu,† Xian Lin,† Hua Ying Chen,§ Jin Quan Chen,‡ Zuanming Jin,*,† Jianhua Xu,‡ and Guo-hong Ma*,† †

Department of Physics, Shanghai University, 99 Shangda Road, Shanghai 200444, China State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China § Harbin Institute of Technology (Shenzhen), HIT Campus, Xili University Town, Shenzhen 518055, China ‡

ABSTRACT: The exciton effect in two-dimensional (2D) transition metal dichalcogenides (TMDs) plays a dominating role in describing the optical and optoelectronic properties. However, the interplay between the excitons and free carriers has not yet been understood upon photoexcitation in 2D TMDs. Here, we first present a study of the dynamical interplay of excitons and unbound electron−hole pairs using time-resolved terahertz (THz) spectroscopy (TRTS) in a few-layer WS2 laminate. Our experimental results demonstrate that the Auger recombination is observed in the relaxation process of the mobile charge carriers rather than that of excitons upon photoexcitation. The transient complex THz photoconductivity spectroscopy of WS2 is well described by the Drude−Lorentz model of free carriers modulated by the exciton polarization field. Our results provide a comprehensive understanding of nonequilibrium carrier kinetics (both excitons and free carriers) in WS2 laminate, and should be applicable to other 2D systems.

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INTRODUCTION Atomically thin transition metal dichalcogenides (TMDs, e.g., MoS2, WS2, MoSe2, and WSe2) have attracted much interest as a new class of two-dimensional (2D) material expected for use with graphene. Because of their unique properties, such as high mechanical strength and flexibility,1 valley control spin polarization,2−4 indirect-to-direct band gap transition,5 and sensitive dependence on the surrounding dielectric media,6,7 TMD materials will be promising for potential applications in optoelectronics and electronics,8 including field-effect transistors (FETs),9 integrated circuits,10 photovoltaic elements,11−13 and so on. One of the significant properties of these atomically thin 2D semiconductors is the extraordinary exciton effect.14−19 In 2D materials, strong quantum confinement and reduced dielectric screening result in substantial Coulomb interactions among the carriers, leading to tightly bound excitons even at room temperature.6,14,20 The resulting formation of bound electron− hole pairs, or excitons, can dominate the optical and chargetransport properties.21 In addition, reduced dimensionality can also enhance many-body interactions, including Auger recombination20 or exciton−exciton annihilation.22,23 Hence, 2D materials form an ideal model system to study many-body interactions of electronic quasiparticles in the nonequilibrium excited state. Previously, many-body interaction processes have been intensively investigated in quantum wells,24 quantum dots,25,26 and carbon nanotubes.27,28 Until now, one of the most studied TMD materials either for basic photophysics research or for potential applications in optoelectronic devices is MoS2; however, other compounds may be better adapted to certain practical uses or better appropriated for revealing the underlying relaxation process.29 As compared to MoS2, WS2 shows similar peak widths © XXXX American Chemical Society

of the A and B exciton with about twice the A−B energy separation,30 which should enable one to settle previously undetected photoexcitation dynamics.29 As far as we have noted, only a few literature pieces have studied the 2D TMDs using optical pump−THz probe spectroscopy;8,20,31,32 however, the time evolution of the photoinduced conductivity of WS2 has not been reported yet. Additionally, it is still a matter of debate whether exciton−exciton annihilation occurs upon photoexcitation in WS2.22,33,34 Here, we first employ time-resolved THz spectroscopy (TRTS) to investigate directly the dynamical interplay of photogenerated excitons and unbound electron−hole pairs in large area chemical-vapor deposition-grown few-layer WS2 laminate. The THz electric field transmitted through the photoexcited sample is detected in time, and, therefore, both the amplitude and the phase spectral information are accessible via Fourier transform within the frequency measurement range of the THz probe.35 In the strongly confined WS2 studied here, the exciton response was characterized by a large optical polarizability, which is directly proportional to the imaginary part of the photoconductivity.36 In addition, the real part of the photoconductivity, which dominated the photoinduced THz transmission magnitude, is mainly contributed by the photoexcited free carriers in the WS2 film. Hence, using time-resolved THz spectroscopy and appropriate modeling, the THz response of excitons and (quasi) free charge carriers can be distinguished.37,38 The subpicosecond and ∼22 ps decays of THz-probed real Received: June 1, 2017 Revised: August 4, 2017 Published: August 25, 2017 A

DOI: 10.1021/acs.jpcc.7b05345 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the pump-induced THz transmission change can be obtained by placing the chopper on the pump beam, and stage II is used. The signal was collected with a lock-in amplifier phase locked to an optical chopper that modulated either the THz generation arm or the pump beam at a frequency of 500 Hz. The spot size of the terahertz beam on the sample position was 2 mm, whereas the spot size of the pump beam on the sample was 5 mm. All measurements were performed at room temperature. The entire THz beam path was enclosed and purged with dry nitrogen environment to avoid water absorption.

photoconductivities are attributed to the phonon emission and Auger recombination process, respectively. However, the exciton−exciton annihilation process has not been observed in the dynamics of photoinduced imaginary conductivity. The transient complex conductivity of WS2 samples is able to be described by a Drude−Lorentz model with a restoring force, which indicates that the THz response of free carriers is modulated by the exciton polarization field.

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EXPERIMENTAL SECTION Sample Preparation. The WS2 laminate was prepared on fused quartz substrates by chemical vapor deposition.39 Briefly, tungsten was prepared by tungsten oxide (WO3) powder and sulfur powder. First, the WO3 powder was positioned on a silicon substrate placed in the center of the furnace and nearly to the fused quartz substrate, which was used as the growth substrate. The sulfur powder then was placed next to WO3. The reaction temperature of the chamber was increased to 1000 °C min−1 at the rate of 15 °C min−1. While the temperature was kept for 30 min, sulfur was evaporated and reacted with WO3. Afterward, the heater of the furnace was turned off, and the sample was left in the chamber to cool to room temperature. Time-Resolved THz Spectroscopy. Time-resolved terahertz spectroscopy (TRTS) was driven by a Ti:sapphire regenerative amplifier (Spectra-Physics, Spit-fire), operating at the repetition rate of 1 kHz, with a central wavelength of 800 nm and pulse duration of 120 fs. The laser was split into three beams: the first beam was the pump beam, which is frequency doubled with a BBO crystal for the ultrafast pump, and the pulse delay time (τp) is controlled via a translation stage I; the second beam was the THz generation beam, where the THz pulse is generated via optical rectification on a 1 mm-thick (110)-oriented ZnTe nonlinear crystal. The THz radiation was collimated and focused by a pair of off-axis parabolic mirrors onto the sample, and then the THz signal taken along the sample information was collimated and focused by another pair of off-axis parabolic mirrors onto another 1 mm-thick (110)-oriented ZnTe crystal. The third beam is the sampling beam, where the THz electric field waveform is mapped with a standard electro-optic sampling technique.40 The time delay of sampling pulse (τd) is scanned using a translation stage II. To measure the THz transmission spectroscopy of the sample without photoexcitation, the chopper was placed on the THz generation beam by blocking the pump beam. Upon pump beam photoexcitation centered at 400 nm,

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RESULTS AND DISCUSSION

Figure 1a and b shows the absorption and Raman spectroscopy of the WS2 laminate, respectively. The absorption spectroscopy indicates the A, B, and C exciton resonance peaks appeared at 1.94, 2.35, and 2.7 eV, respectively. A and B are exciton resonances corresponding to transitions at the “K” point from the two spin-split valence bands to the conduction bands. Peak C located around 2.7 eV is recognized as the excitonic transition from multiple points near “Γ” point of the Brillouin zone. We observed two characteristic Raman modes corresponding to the E12g (in-plane vibration) and A1g (out-of-plane vibration) consistent with the previous report.22 The frequencies of these two modes are around 350 and 419 cm−1, respectively. A frequency difference between E12g and A1g modes is 69 cm−1, which corroborates the laminate consists of 3−4 layers.22 The carrier dynamics in photoexcited 2D WS2 was probed using time-resolved terahertz (THz) spectroscopy (TRTS) with the pump pulse photoenergy of 3.1 eV. The standard TRTS measurement was carried out in a transmission configuration.40 As terahertz spectroscopy was sensitive to not only mobile charge carriers but the exciton polarization,36 TRTS provided the nature of photoinduced charges (excitons or free carriers) and the photoinduced intrinsic charge carriers’ mobility. Figure 2a shows the time-domain terahertz spectroscopies of WS2 laminate. The black curve represents a typical measurement of a single-cycle THz pulse transmitted through WS2 laminate, Eref(t). The pumpinduced modulation of the THz electric field transmission is recorded as ΔE(t, Δt) = Epump(t, Δt) − Eref(t). A typical ΔE(t, Δt) = 3 ps using 400 nm optical excitation, magnified by a factor of 5, is plotted in red in Figure 2a. The attenuation in peak magnitude of the THz waveform is proportional to the photoinduced real component of photoconductivity, mostly arising from the effect of the free charge carriers. The photoinduced

Figure 1. (a) UV−visible absorption spectroscopy of the WS2 laminate. Peaks A, B, and C denote the exciton resonances corresponding to transitions from valence bands to conduction bands. (b) Raman spectroscopy of the WS2 laminate. The splitting between the peaks of E12g and A1g indicates the layer number. B

DOI: 10.1021/acs.jpcc.7b05345 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. (a) The transmission of THz electric-field Eref(t) and the photomodulation of the THz waveform ΔE (t, Δt = 3 ps) in the WS2 film. (b) Pump−probe scans are performed at the peak and zero-crossing of THz waveform, corresponding to the real and imaginary photoconductivity of WS2. Inset: Normalized pump−probe scans of real Δσ and imaginary Δσ of WS2, respectively. The solid lines are fitted with triexponential model (real conductivity) and biexponential model (imaginary conductivity), respectively. (c and d) Transient photoinduced real and imaginary conductivity of the WS2 laminate at room temperature measured at different pump fluences. The solid lines are fitted with triexponential model (real conductivity) and biexponential model (imaginary conductivity), respectively. Inset: Normalized dynamics of real and imaginary photoconductivity of WS2 laminate measured with various fluences, respectively.

the imaginary conductivities reach the peak (or valley) value simultaneously within 4 ps, indicating the initial excitation of free carriers quickly forms excitons on the time scale within 4 ps.32 It is also found that the decay of the real photoconductivity is much slower than that of the imaginary one. This means that the lifetime of the photoinduced exciton is significantly shorter than that of the mobile charge carriers. It is also seen that the real photoconductivity remains nonzero at the delay time when the imaginary photoconductivity is complete recovery, indicating the existence of mobile charge carriers over a longer period of time in WS2 laminate. To further understand the relaxation dynamics of charge carriers and exciton after photoexcitation, the dynamics of the real and imaginary photoconductivity are studied with various pump fluences. Figure 2c and d shows the typical time-resolved real and imaginary photoconductivity dynamics of WS2 sample with different pump fluences. As shown in Figure 2c, the peak kinetics of THz waveform with a triexponential fit reveals that the real photoconductivity drops about 40% within a fast component τ1 (1−2 ps), and then drops nearly 50% within a slow component τ2 (∼22 ps), and at the last component with ratio of near 10% it takes places within a very slow component τ3 (∼107 ps) and an offset with lifetime longer than 1 ns. The normalized timeresolved real photoconductivity dynamics in WS2 laminate with various fluences is shown in the inset of Figure 2c. The initial subpicosecond process is almost independent of the excitation density, while the slow processes become faster upon higher excitation fluence. As shown in Figure 2d, the phase shift kinetics of the THz waveform with a biexponential fit reveals that the absolute value of imaginary photoconductivity recovers by 45%

phase shift of THz waveform, that is, the change in THz electric field at the zero-crossing point, is proportional to the photoinduced imaginary component of photoconductivity, which contains contributions from polarization effects of bound charges, such as excitons.35,36,41 By varying the delay between the 400 nm optical pulse and the THz probe, the transient of photoinduced terahertz spectroscopy corresponding to the photoinduced real and imaginary conductivity can be mapped out through measuring the photoinduced peak absorption and phase-shift of the THz waveform, respectively. First, we focus on the sheet conductivity change Δσs(t), which can be obtained from the photoinduced peak attenuation or the time-shift in the THz electric-field amplitude (Eref(t)) as a function of pump−probe time delay:42,43 Δσs (t ) = −

1 + n ΔE(t ) Z0 Eref (t )

(1)

where n = 1.95 is the THz refractive index of the fused silica substrate, and Z0 = 377 Ω is the impedance of free space. Figure 2b shows the dynamics of the real and imaginary conductivities in WS2 laminate. The normalized dynamics of real and imaginary conductivity are shown in the inset of Figure 2b. A small hump about 10 ps after the main peak is due to the reflection of the pump pulse at the substrate−air interface. The solid lines are triexponential (real conductivity) and biexponential (imaginary conductivity) fitting, respectively. It should be noted that purely imaginary conductivity is evidence of the formation of bound charges, that is, exciton in WS2 laminate.36 Specifically, as shown in the inset of Figure 2b, both the real and C

DOI: 10.1021/acs.jpcc.7b05345 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. (a) Relaxation components τ1, τ2 of triexponential models of the dynamics of photoinduced real conductivity displayed as a function of F. (b) Relaxation components τ1, τ2 of biexponential models of the dynamics of photoinduced imaginary conductivity displayed as a function of F.

within a fast component τ1 (1−2 ps), and then a further 55% within a slow component (∼16 ps). The normalized timeresolved imaginary photoconductivity dynamics in WS2 laminate with various fluences is shown in the inset of Figure 2d. Both relaxation processes display pump fluence independent behavior. Next, we studied the role of the relaxation components. Figure 3 shows fast (τ1) and slow (τ2) components of the photoinduced real conductivity (a) and the photoinduced imaginary conductivity (b) as a function of the pump fluence, respectively. As shown in Figure 3a, the fast component τ1 displays independent of the pump fluence, which rules out the relaxation of Auger-assisted trapping of carriers in “fast” traps. If the defect-trapped process governs the initial decay, the magnitude of τ1 should decrease as the pump fluence increases.31,44 A previous study of the initial carriers’ relaxation time (hundreds of femtoseconds) in a few-layer MoS2 shows that phonon emission dominates the fast relaxation process. Hence, the fast component τ1 may be mainly dominated by the electron−phonon scattering process,46 because phonon-mediated relaxation time is either independent or increases with the pump intensity due to the hot phonon effect.45,46 On the other hand, the component τ2 decreases as the pump fluence increases, which indicated that it is dominated by a many-body interaction, such as the Auger process.20 The magnitude of τ3 is also seen to be independent of the excitation density, and we assign the τ3 as the electron−hole recombination, in which the electrons and holes are captured by defects via Auger process47 or from the recombination of unbound electron−hole pairs in a confined volume.48 The offset component (>1 ns) is assumed to come from the intermediate midgap trapping.32 In contrast, dynamics of imaginary conductivity shows only two processes, both of which components are independent of the excitation fluence. We assigned the fast component (1−2 ps) as the exciton−phonon coupling,46 and the slow one (∼16 ps) as the exciton lifetime. The exciton lifetime is approximately in accordance with the previous report on a monolayer WS2.34The exciton−exciton annihilation has not been observed in the exciton dynamics as the decay process is independent of the excitation fluence, which is consistent with the previous report in monolayer WS234 and trilayer WS2.22 Further information can also be gained from the complexvalued THz frequency-resolved photoconductivity spectra under measuring different pump delay times, Δt. In the frequencyresolved conductivity spectra of the WS2 film, positive real conductivity is observed, while the imaginary conductivity contains both positive and negative value. Specifically, the imaginary component is negative at low frequencies but becomes

positive at the frequency higher than v0. It is noted that the real conductivity reaches its maximum at the same zero-crossing frequency, v0. Similar behavior has been observed on a semiconducting carbon nanotube (CNT) agglomerated film49 and isolated CNTs.50 They attributed this behavior to a combination of quasi-free charge carriers and charge carriers bound in exciton, or the exciton transition, respectively. The THz conductivity of samples, which are doped with a lot of defects, may be described by a combination of the Drude−Smith model and the Lorentz model, such as graphene oxide.51 However, there are fewer defects in our chemical vapor deposition-grown WS2 laminates than in GO. Therefore, we describe the observed responses of WS2 with the Drude−Lorentz model that has successfully been used to describe complex conductivities of semiconductor nanomaterial.38 The positive real conductivity reveals the photoexcitation of mobile charge carriers. The purely imaginary conductivity responses in the THz spectral range are evidence of the presence of a restoring force in the carrier motion, which indicates the charge carriers localized or “bound” in excitons.36,52 Referring to the Drude−Lorentz model, the complex photoconductivity can be expressed as σ(ω) =

ε0ωp2τ 1 − iτω + iτω0 2 /ω

(2)

where ωp is the plasma frequency, τ is the average scattering time, ε0 is the vacuum permittivity, ω0 is the angular frequency of the oscillatory response, and the plasma frequency ωp is related to the photocarrier density by ωp2 =

e 2N ε0m*

(3)

where N is carrier density, e is the elemental charge, and m* is the carrier effective mass. As seen in Figure 4, the photoconductive response of WS2 can be well reproduced with the Drude−Lorentz model, eq 2. The fitting parameters are presented in Tables 1 and 2, respectively. As pump fluence increases, the plasma frequency (ωp) is varied from 2.12 × 1010 to 4.76 × 1010 Hz (Table 1). As described in eq 3,ωp ∝ √N, the mean carrier momentum scattering time (τ) is found to decrease from 71 to 30 fs as the pump fluence increases. The magnitude of τ obtained in the present study has a value similar to the previous report in a chemical-vapor deposition grown single-layer graphene.53,54 The frequency of the oscillatory response (v0) increases with the carriers’ density. Similar conclusions can be drawn from Table 2. The extracted frequency (v0) of Lorentzian resonance is 1.35−2.12 THz, which D

DOI: 10.1021/acs.jpcc.7b05345 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) Complex probe frequency-resolved photoconductivity of WS2 film measured at Δt = 3 ps after photoexcitation at various fluences of 30, 40, and 60 μJ/cm2, respectively. (b) The complex frequency-resolved photoconductivity with 50 μJ/cm2, measured at various pump delays of Δt = 5, 15, and 25 ps, respectively. The solid lines are Drude−Lorentz model fitting.

carriers modulated by the exciton polarization field. Because of the near-field interactions, the linear array of closely spaced excitons can therefore be viewed as a chain of interacting dipoles, which supports traveling polarization waves. The exciton polarization field offers a restoring force acting on the free carriers, inducing a surface electron−hole plasma depletion or accumulation field, thus causing the plasma resonance.38 Hence, the interaction between the excitons and free carriers is further verified by the frequency-resolved conductivities of WS2 laminate.

Table 1. Fit Parameters from the Probe FrequencyDependent WS2 Conductivity Fitted to the Drude−Lorentz Model with Various Pump Fluences (30−60 μJ/cm2) and Delay Time of Δt = 3 ps (Equation 2) fluence (μJ/cm2)

ωp (×1010 Hz)

τ (fs)

v0 (THz)

30 40 60

2.12 ± 0.24 3.68 ± 0.06 4.76 ± 0.7

71 ± 9 35 ± 7 30 ± 2

1.35 ± 0.3 1.72 ± 0.6 2.12 ± 0.5

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CONCLUSIONS In summary, we have observed ultrafast photoconductivity response in WS2 laminate. In this way, the dynamics of excitons and the unbound electron−hole pairs can be distinguished using time-resolved terahertz (THz) spectroscopy. Frequency-dependent complex THz conductivity indicates that the free carriers are influenced by the exciton polarization field. The measured timeresolved evolution of photoexcited carriers is useful for the understanding of the basic optoelectronic properties in 2D TMDs and provides new opportunities for developing novel optoelectronic and excitonic devices based on this material.

Table 2. Fit Parameters from the Probe Frequency-Dependent WS2 Conductivity Fitted to the Drude−Lorentz Model with Fixed Pump Fluence of 50 μJ/cm2 and Measured at Δt = 5, 15, and 25 ps (Equation 2) Δt (ps)

ωp (×1010 Hz)

τ (fs)

v0 (THz)

5 15 25

2.35 ± 0.24 1.4 ± 0.07 1 ± 0.1

56 ± 6 73 ± 7 78 ± 8

1.9 ± 0.5 1.67 ± 0.35 1.5 ± 0.03

is dependent on either the pump fluence (Table 1) or the pump delay time (Table 2). We note that the resonance can hardly have resulted from the exciton resonance on account of that the large binding energy of exciton is about 0.32 eV in WS2, which is far outside the frequency measurement range (0.4−2.5 THz) of our THz spectroscopy.19 In addition, the resonance frequency cannot be assigned to the electronic transition in a bound complex, such as a 1s−2p intraexcitonic transition.50 In fact, the resonance frequency of the intraexciton transition is expected to be independent of the photocarrier density, which obviously is not consistent with our observation. Some previous studies of 2D TMD materials also show non-Drude model of frequencydependent THz conductivity. Liu et al. failed to fit the conductivity in a monolayer MoS2 with a pure Drude model either before or at different delays after pump excitation.32 The THz response was interpreted to be caused by many factors such as tunneling transport through the grain boundary or the low initial doping intensity. Docherty et al. reported that the THz complex photoconductivity spectra of MoS2 and WSe2 have a free carrier (Drude type) response combined with stronger Lorentzian resonances.31 The resonance frequencies were mainly attributed to trion and neutral exciton resonances. Instead, we infer the resonant frequency (v0) in the THz-range is contributed by free

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 86-21-66132513. E-mail: [email protected]. *E-mail: ghma@staff.shu.edu.cn. ORCID

Jin Quan Chen: 0000-0003-0652-1379 Guo-hong Ma: 0000-0002-3972-5012 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, nos. 11674213, 11604202 and 61735010) and the Research Innovation Fund of the Shanghai Education Committee (14ZZ101). Z.J. is thankful for the Young Eastern Scholar (QD2015020) at Shanghai Institutions of Higher Learning, the Colleges, Universities Young Teachers Training Funding Program (ZZSD15098), and the “Chen Guang” project (16CG45) supported by the Shanghai Municipal Education Commission and Shanghai Education Development Foundation. E

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