Ultrafast Energy Dissipation via Coupling with Internal and External

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Article

Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS 2

zhen chi, Huihui Chen, Zhuo Chen, Qing Zhao, Hailong Chen, and Yuxiang Weng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02354 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2

Zhen Chi,†,§Huihui Chen,‡ Zhuo Chen,‡ Qing Zhao,§ Hailong Chen,*,† and Yu-Xiang Weng†,‖

†

Beijing National Laboratory for Condensed Matter Physics, CAS Key Laboratory of

Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

Department of Materials Physics and Chemistry, Beijing Key Laboratory of

Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology Institution, Beijing 100081, China §

Center for Quantum Technology Research, School of Physics, Beijing Institute of

Technology, Beijing 10081, China ‖

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing

100049, China * Corresponding author: Hailong Chen (email: [email protected])

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ABSTRACT Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by non-radiative energy dissipation, has greatly limited practical applications. To reveal the details for non-radiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS2. With this method, the many-body effects, occupation effects and phonon dynamics are clearly identified. In particular, thermalization of the MoS2 lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within 10 to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (~9 ps) is mainly dissipated from the MoS2 surfaces to external phonons. KEYWORDS: 2D materials, ultrafast spectroscopy, electronic and phonon dynamics, energy dissipation, temperature sensor


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Two-dimensional transition metal dichalcogenides (TMDCs), such as MoS2, WS2, MoSe2, WSe2, etc., have gained tremendous attention in recent years due to their intriguing mechanical, electronic and optical properties.1-5 They offer great opportunities for fundamental and technological research in a variety of fields including

electronic

and

optoelectronic

devices,6-11

photocatalysis

and

electrocatalysis,12-18 energy storage,19-23 spin and valley physics,24-27 and so on. In all of these applications, non-radiative electronic energy dissipation in single- to few-layer TMDCs always plays a significant role. In particular, the relatively low quantum yield of current TMDC-based optoelectronic devices mainly originates from the non-radiative recombination of photo-induced carriers with the excess energy inevitably converted into heat,28 which can also lead to premature failure and shortening of the service life for the device. In general, two non-radiative energy channels should be carefully considered after photoexcitation of layered TMDCs (Figure 1a, curved arrows): (1) electron-phonon scattering during the ultrafast cooling of hot carriers and subsequent formation of bound carriers (e.g., excitons), and (2) non-radiative recombination of electron-hole pairs. A thorough study of these processes should be of central importance for revealing details of the electronic energy dissipation pathways in TMDCs. Ultrafast pump-probe spectroscopy has been widely used and demonstrated as a powerful tool for the study of various carrier dynamics in TMDCs, including carrier recombination,29-31 exciton-exciton annihilation,32,33 ultrafast charge transfer in TMDC heterostructures,34-36 spin-valley dynamics,26,37 etc. Despite such recent


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progress, detailed non-radiative energy dissipation dynamics in TMDCs following ultrafast photoexcitation remains poorly understood. One of the main reasons for this lack of understanding is that traditional optical pump-probe studies of single- to few-layer TMDCs are generally based on the measurement of resonant or near-resonant interband excitonic nonlinearities by tuning the probe wavelength close to the exciton line. The resulting data are usually difficult to interpret quantitatively,30 since, in most cases, there exists plenty of photo-induced dynamics (as mentioned above) that can be temporally mixed. To address this issue, we herein carry out a comprehensive ultrafast spectroscopy study for solution-processed few-layer MoS2 with the detection wavelength ranging from visible to near-infrared to mid-infrared. As schematically illustrated in Figure 1a, photo-induced unbound (or bound) carriers are first prepared by non-resonant (or resonant) excitation of femtosecond pump pulses. Then, the dynamics of various carriers are monitored by recording absorption spectra at different spectral regions with a controlled time delay between pump and probe pulses. In addition to traditional visible pump-probe spectroscopy, both near-infrared (NIR) and mid-infrared (MIR) pulses are also utilized in this work as optical probes. Based on the different response of photo-induced carriers on different probe photon energies in the near- to mid-infrared region, the dynamics of various kinds of bound and unbound carriers in few-layer MoS2 can be investigated.30,36,38-41 Specifically, the NIR spectrum is affected by the intraband absorption due to both free carriers and excitons,30 as well as the bleaching of optically active midgap defect states.38 In contrast, the MIR spectrum,


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if not close to the spectral range of intraexcitonic resonances,39,40 is only sensitive to free carriers and weakly bound carriers with binding energies well below the probe energy.36 Comparing all these results with those obtained from traditional visible pump-probe measurements, the photo-induced carrier dynamics in few-layer MoS2 are specifically described in detail. Most importantly, the underlying thermal effect after photoexcitation of MoS2 nanosheets is therefore clearly revealed. In particular, one can directly monitor the temporal evolution of lattice temperature. Further results demonstrate that the excess energy from the cooling of hot carriers and the formation of bound carriers plays a major role in the increase in lattice temperature, while the excess energy from defect-mediated carrier recombination mostly dissipates from the surface of MoS2 nanosheets during the recombination process. RESULTS AND DISCUSSION Photo-Induced Red- and Blueshift of the Exciton Resonance. Few-layer MoS2 nanosheets were prepared by ion exchange reaction in the solution phase,42 which is a low-cost and mass production method compared with mechanical exfoliation and chemical vapor deposition. For a better comparison, two samples containing MoS2 nanosheets either dissolved in ethanol or coated onto a calcium fluoride (CaF2) window were measured in this work. The dominated thickness of the nanosheets is approximately 6-7 layers, which was confirmed by both Raman spectroscopy and atomic force microscopy (see Supporting Information Figure S1). Except for the measurement of temperature-dependent absorption spectra, all the experiments were carried out at room temperature and under ambient conditions. The absorption spectra


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for the two samples show almost the same features (Figure 1b), indicating the electronic structure of MoS2 was barely altered during the coating process. Both spectra show characteristic A- and B-exciton resonances of approximately 1.83 eV and 1.98 eV, respectively, which are ascribed to excitonic transitions occurring at the K/K' points of the k-space.43,44

Figure 1. Schematic diagram of the multiple relaxation pathways for the photo-induced carriers and the observed ultrafast spectra in the visible region of the few-layer MoS2 nanosheets. (a) Schematic of the ultrafast pump-probe spectroscopy with optional visible (Vis), near-infrared (NIR) and mid-infrared (MIR) detection irradiation. Dashed and solid straight arrows denote possible optical transitions caused by the pump and probe pulses, respectively. Solid and dashed parabolas schematically show the dispersion states with a principal quantum number n = 1, 2, and so forth and the carrier continuum state. Curved arrows indicate two major electronic energy dissipation processes, as discussed in the main text. (b) Absorption spectra for few-layer MoS2 nanosheets in ethanol (solution) and on a calcium fluoride substrate (film). Red and blue shaded areas indicate the pump photon energies for resonant and non-resonant excitations, respectively. (c) Transient absorption spectra for the sample in ethanol at selected time delays following photoexcitation at 1.83 eV (left, red lines) and 3.10 eV (right, blue lines) with a pump fluence of 64 and 77 µJ/cm2, respectively. Absorption spectrum at a time delay of -0.4 ps representing the response prior to excitation is included in each panel for


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comparison (gray lines). The broad background in each transient absorption spectrum has been removed.

Visible pump-probe measurements, which focus on the optical response at the Aand B-exciton resonances after photoexcitation, were first carried out for the two different samples. The photon energy of the pump pulses was tuned to be 1.83 eV for resonant excitation and 3.10 eV for non-resonant excitation (shaded areas in Figure 1b), and the pump fluence varied from 37 to 150 µJ/cm2 and 44 to 180 µJ/cm2, respectively. The excitation at 1.83 eV, which is close to the MoS2 A-exciton absorption maximum, creates bound carriers (e.g., excitons and weakly bound electron/hole pairs36), while that at 3.10 eV, which is far above the fundamental bandgap, predominantly induces free charge carriers.36,39 Figure 1c shows the transient absorption spectra for MoS2 in ethanol at different time delays following photoexcitation at 1.83 and 3.10 eV. The absorption spectrum measured at a time delay of -0.4 ps representing the response prior to excitation is also plotted in each panel for comparison (gray lines). The broad background in each transient absorption spectrum was removed by fitting with a quadratic polynomial. The pump pulses with a fluence of 64 µJ/cm2 for excitation at 1.83 eV and 77 µJ/cm2 for excitation at 3.10 eV generate electron-hole pairs with almost equal density per MoS2 layer (~8.5×1012 cm-2, see Supporting Note 1), below the Mott threshold for the MoS2 nanosheets.45,46 All the transient absorption spectra exhibit unambiguous bleaching of the exciton resonances following ultrafast excitation, which fully recover


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after hundreds of picoseconds. Interestingly, a blueshift of the exciton resonance energy upon excitation at 1.83 eV is clearly observed (Figure 1c, left panel), while a pronounced redshift appears when the pump photon energy is turned to 3.10 eV (Figure 1c, right panel). The same phenomenon was also observed in measurements of MoS2 nanosheets on a CaF2 substrate (see Supporting Information Figure S2). To clearly identify the time-dependent exciton resonance energy shift, a line shape analysis was carried out by fitting all the measured transient absorption spectra with Lorentzian functions (see Supporting Information Figure S3). Three fitted peaks are identified that are attributed to the trion (A-, ~1.79 eV), A-exciton (~1.83 eV) and B-exciton (~1.97 eV).47,48 Figure 2a shows the time-dependent energy shift ∆E for the MoS2 A-exciton in ethanol after resonant (1.83 eV) and non-resonant (3.10 eV) excitation with different pump fluences. Following non-resonant excitation, a negative ∆E corresponding to a redshift of the resonance energy immediately appears, the value of which increases significantly at higher pump fluence. Even after tens of picoseconds, a notable redshift of the exciton resonance energy is still observed. In contrast, after resonant excitation, a positive ∆E (blueshift) reaches its maximum immediately, followed by a fast decay within several picoseconds. Only a slight redshift can be observed at tens of picoseconds, which becomes more obvious at higher pump fluence. The same behavior was also observed for the time-dependent energy shift for the B-exciton (see Supporting Information Figure S4).

Figure 2. Ultrafast dynamics after photoexcitation of MoS2 nanosheets in ethanol


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measured at different probe photon energies. (a) Time-dependent energy shift ∆E for the MoS2 A-exciton after excitation at 1.83 eV and 3.10 eV with different pump fluences. (b) Temporal evolution of the excitation-induced transmission change for the MoS2 nanosheets detected at 1.31 eV under excitation at 1.83 eV (140 µJ/cm2). (c) Temporal evolution of the excitation-induced absorption change for the MoS2 nanosheets detected at 0.25 eV under excitation at 1.83 eV (110 µJ/cm2) and 3.10 eV (110 µJ/cm2). For (a)-(c), the dots are the data, and the curves show multi-exponential fitting that includes consideration of the instrument response function (~150 fs). (d) Schematic illustration of the overall scenario after photoexcitation of few-layer MoS2 for which the dynamic process can be described roughly in three stages.

We first discuss the opposite energy shifts observed at the beginning after resonant and non-resonant excitation. As previously reported, the presence of photo-induced carriers in MoS2 leads to Pauli-blocking of the occupied states, which results in bandgap renormalization and plasma screening of the Coulomb interaction.49 The former effect usually causes a redshift of the exciton resonance energy due to band gap shrinkage, with the latter leading to a blueshift that can be attributed to a decrease in the exciton binding energy.46 Therefore, the overall shift in the resonance energy can be ascribed to a competition between band gap shrinkage and decreasing exciton binding energy.49-51 Our experimental results indicate that the former is relatively more significant in few-layer MoS2 under non-resonant excitation, where an abundance of free charge carriers are generated. In contrast, the reduced exciton


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binding energy plays a dominant role after resonant excitation, which mainly leads to the creation of bound electron-hole pairs (e.g., excitons). However, the observed redshift at tens of picoseconds for both the resonant and non-resonant excitations cannot be explained by the competition between these two effects. As discussed in the following sections, this redshift arises from the elevation of the lattice temperature following photoexcitation, which shifts the exciton peak position to lower energies. Electronic and Phonon Dynamics after Ultrafast Excitation. To clarify the various carrier dynamics in few-layer MoS2 and to therefore extract the underlying thermal effect after ultrafast excitation, the time scales for the different dynamic processes should be first clearly identified. As shown in Figure 2a, the temporal evolution of ∆E after 3.10 eV excitation can be described roughly in three stages that are marked with different colors: (I) a fast decrease and recovery of ∆E within 2 ps, (II) a following recovery for ∆E, with a time constant of approximately 15 ps, and (III) finally, a slow recovery of ∆E that lasts for more than hundreds of picoseconds. All three time scales show no significant change with increasing pump fluence. Similarly, after 1.83 eV excitation, the blueshift of the exciton resonance rapidly reaches its maximum, followed by a fast (~2 ps, stage I) and slow decay (~8 ps, stage II). Then, the ∆E turns negative and lasts for hundreds of picoseconds (stage III). All the time scales are also found to be independent of the pump fluence. To explicitly assign these processes, we further successively tuned the probe photon energy to 1.31 eV (NIR) and 0.25 eV (MIR). As shown in Figure 2b, the temporal evolution for the excitation-induced transmission change at 1.31 eV shows two bleaching recovery


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components, with time constants of 2 ps (~80％, stage I) and 9 ps (~20％, stage II). Fitting details are provided in Supporting Note 2. By comparison, a rapid increase in the transient absorption signal after photoexcitation is observed at the MIR region (Figure 2c), followed by a fast decay within 2 ps (stage I) for both resonant and non-resonant excitation. As mentioned above, the MIR detection signal is predominantly due to the absorption of free charge carriers as well as weakly bound electron/hole pairs. Since the photon energy of 0.25 eV is well below the excitonic-binding energy for the layered MoS2, the tightly bound carriers, such as excitons and trapped carriers, can barely absorb the probe photons and act as a charge-neutral insulating gas.30,52 Therefore, the signal decays shown in Figure 2c mainly reflect the rapid conversion of photo-induced free carriers (3.10 eV excitation) and weakly-bound electron/hole pairs (1.83 eV excitation) into tightly bound carriers.36 Multi-exponential fitting of the data reveals two decay constants of approximately 0.5 ps (64％) and 3 ps (36％) for both resonant and non-resonant excitation, which are almost independent of the pump fluence (see Supporting Information Figure S5 and Table S1). The faster decay component can be attributed to the ultrafast formation of tightly bound excitons,36 which is consistent with previous observations for a WSe2 monolayer.39 Decay dynamics with a time constant of ~3 ps were also observed in previous studies for monolayer and few-layer MoS2, which was ascribed to a fast trapping of carriers by surface trap states.29,30 In addition, compared with excitation at 1.83 eV excitation, an extra exponential rise with a time constant of 90 fs is clearly observed under


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excitation at 3.10 eV (see Supporting Information Figure S6 and Table S1). This reflects the initial relaxation process for the non-resonant excitation-induced hot electron-hole gas via strong electron-electron scattering.53,54 As a consequence, most of the photo-induced carriers form tightly bound excitons and trapped carriers within 2 ps (stage I). Furthermore, the bleaching signal observed in the NIR region (Figure 2b) reflects the subsequent dynamics. In a previous study,38 a weak absorption band in the 0.8-1.6 eV range was observed in few-layer MoS2, which is due to optical transitions from midgap defect states to the conduction bands. The positive ∆T/T observed immediately after photoexcitation was attributed to decreased defect absorption by the probe pulse due to ionization of the defects by photoexcitation.38 Therefore, the slow decay component on the picosecond time scale (~9 ps) in Figure 2b could be assigned to non-radiative recombination of trapped electron-hole pairs (stage II). So far, the temporal evolutions for the exciton resonance shift ∆E shown in Figure 2a can be clearly described. The observed fast recovery component (~2 ps) for excitation at both 1.83 and 3.10 eV mainly arises from the formation of tightly bound excitons and trapped carriers following ultrafast excitation, and the relatively slow component (~8 ps) after excitation at 1.83 eV arises from non-radiative carrier recombination. As widely reported, defect-mediated non-radiative recombination (Shockley-Read-Hall recombination) and Auger-type biexcitonic recombination at higher excitation powers dominate the decay channels for layered MoS2.30,32,38,55-57 In our case, the former recombination mechanism plays a dominant role, since all the


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time constants discussed above are nearly independent of the pump fluence. However, the ~15 ps recovery component for the negative ∆E in stage II for excitation at 3.10 eV (Figure 2a) cannot be simply attributed to the non-radiative recombination process, since the dominant bound electron-hole pairs existing in this stage should result in a positive ∆E, as discussed above. To explain this issue, the thermal effect should be carefully considered here, which has been previously demonstrated to play a particularly important role in pump-probe experiments.51,58 Specifically, with the formation of bound carriers from a non-resonant excitation-induced hot electron-hole gas, the excess energy is inevitably dissipated into the lattice via electron-phonon scattering. The subsequent increase in the local temperature will shift the exciton resonance to lower energies, in accordance with the temperature-dependent shift observed for the band gap. To verify this effect, temperature-dependent absorption spectra were collected for MoS2 nanosheets in ethanol. As shown in Figure 3a, the resonance energies for both the A-exciton and B-exciton monotonically decrease with increasing temperature, which is consistent with the above conclusion. Therefore, the heating of the lattice by the hot electron-hole gas via electron-phonon coupling is responsible for the negative ∆E observed in stage II under excitation at 3.10 eV (Figure 2a), and the slow recovery that lasts hundreds of picoseconds in stage III represents the cooling process for the lattice following ultrafast thermalization.

Figure 3. Temporal evolution for the lattice temperature in MoS2 nanosheets after thermalization. (a) Temperature-dependent absorption spectra for few-layer


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MoS2 in ethanol. The broad background in each absorption spectrum has been removed. (b) The exciton resonance energies extracted from (a) are plotted as a function of temperature. (c) & (d) Temporal evolution of the lattice temperature change ∆T in MoS2 nanosheets after excitation at 3.10 eV with pump varying fluence. The data (dots) along with bi-exponential fitting (lines) are extracted from the energy shift of the A-exciton resonance after 5 ps for the MoS2 solution (c) and film (d). Insets of (c) and (d) plot the corresponding temperature increase ∆T under varying pump fluence for a time delay of 10 ps, with all the data (dots) fitted to a linear function (lines).

The same experimental results were obtained for MoS2 nanosheets on a CaF2 substrate (see Supporting Information Figure S7, Figure S8 and Table S2). Based on the above analysis, the time scales of different dynamic processes after ultrafast excitation are clearly identified. For example, the many-body effect such as electron-electron scattering is observed within 100 fs after photoexcitation by measuring the MIR detection signal, and the electron-phonon scattering is demonstrated to occur on the picosecond time scale, which is responsible for the observed redshift of the exciton resonance energy on longer time scales. After photoexcitation of few-layer MoS2, the overall scenario can be roughly described by three stages with different corresponding time scales, as schematically depicted in Figure 2d. Temporal Evolution of the Lattice Temperature after Ultrafast Thermalization.


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From the temperature-dependent absorption spectra for few-layer MoS2 in ethanol (Figure 3a), the exciton resonance energies at different temperatures can be extracted by fitting with Lorentzian functions (see Supporting Information Figure S3). As shown in Figure 3b, the resonance energies for both the A-exciton and B-exciton decrease linearly with increasing temperature from 284 to 352 K. From linear fitting, the temperature coefficients for the A-exciton and B-exciton are obtained as -0.42 and -0.49 meV/K, respectively, which are very close to each other. Similar values lying between -0.26 and -0.5 meV/K have been extracted for various TMDCs from temperature-dependent photoluminescence measurements.59,60 Based on the linear relationship between the temperature and resonance energy for the A-exciton, the excitation-induced temperature increase, ∆T, for MoS2 can be well estimated.51 The temporal evolution for ∆T in MoS2 solution and film after excitation at 3.10 eV with varying pump fluence is shown in Figure 3c and 3d, which was extracted from the corresponding time-dependent exciton resonance shift. Only the data measured after a time delay of 5 ps are shown here, for which the thermal effect dominates the redshift of the resonance energies. The insets of Figure 3c and 3d plot the temperature increases ∆T under varying pump fluence 10 ps after excitation, when most of the electron-hole pairs have recombined. The lattice temperature increase shows a linear relationship with the pump fluence for both the MoS2 solution and film, which is consistent with a previous report.51 The slight deviation in ∆T from a linear relationship for the MoS2 film at 180 µJ/cm2 is probably because the density of the generated electron-hole pairs is too close to the Mott threshold for the MoS2


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nanosheets. All these data for the temporal evolution of ∆T exhibit a bi-exponential decay feature. Table 1 lists the fitted time constants for the decay curves shown in Figure 3c and 3d. The faster decay component can be attributed to a rapid equilibrium of the lattice temperature within the entire nanosheets after ultrafast local thermalization, with the relatively slower component corresponding to the transfer of heat from the MoS2 nanosheets to the surrounding solvent (solution) or underlying substrate (film). The equilibrium time for the lattice temperature for the MoS2 in ethanol (~15 ps) is clearly longer than that for the MoS2 on the CaF2 substrate (~5 ps). This is probably because the MoS2 nanosheets in ethanol are more flexible, retarding the heat dissipation within the entire nanosheets. In contrast, the final lattice cooling time for the MoS2 film (~1100 ps) is much longer than that in ethanol (~550 ps). This could be explained by the different environment of the MoS2 nanosheets in the two samples. The MoS2 nanosheets in ethanol are completely surrounded by solvent molecules to which generated heat can be transferred with high efficiency. For MoS2 nanosheets on the CaF2 substrate, more than half of the MoS2 surfaces are in contact with air or other MoS2 nanosheets, limiting heat transfer efficiency to the CaF2 substrate. These results also confirm that the temporal evolution of the lattice temperature in MoS2 nanosheets after thermalization can be well monitored by using this method.

Table 1. Fitted time constants τ1 and τ2 for the decay curves (∆T) shown in Figure 3c and 3d.


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Pump Fluence

solution

film

τ1 (ps)

τ2 (ps)

τ1 (ps)

τ2 (ps)

44 µJ/cm2

19±2

550±10

4.7±0.2

1100±30

55 µJ/cm2

16±1

520±10

4.7±0.1

1100±20

77 µJ/cm2

16±1

560±10

5.5±0.2

1080±20

110 µJ/cm2

14±1

550±10

6.3±0.2

1060±20

180 µJ/cm2

13±1

520±10

7.1±0.2

1510±30

Rapid Energy Dissipation through Defect-Mediated Carrier Recombination. The temporal evolution of the energy shift ∆E for the MoS2 solution and film excited at 1.83 eV is shown in Supporting Information Figure S9. Similar to the data shown in Figure 2a, a redshift following a blueshift is observed at a time delay of approximately 15 ps and 10 ps for the solution and film, respectively. Furthermore, the redshift becomes more significant with increasing pump fluence, especially for the sample on the CaF2 substrate. It is well known that non-radiative recombination of electron-hole pairs always generates heat in materials via activation of the phonons in the lattice. Thus, the redshift observed at a relatively longer time delay after resonant excitation can be attributed to a thermal effect that is mainly caused by non-radiative recombination. In Figure 4a, the temporal evolution of ∆T is compared for the MoS2 solution and the film under different excitation conditions, with the initial density of photo-induced electron-hole pairs per layer nearly equal in both cases (~1.9×1013 cm-2, see Supporting Note 1). Two interesting features are identified: (1) the maximum ACS Paragon Plus Environment

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temperature increase ∆T for the sample in solution after photoexcitation at 3.10 eV is almost 30 times larger than the ∆T observed following excitation at 1.83 eV (red and green lines), and (2) compared with the data measured in solution, the MoS2 on the CaF2 substrate can reach a higher temperature after excitation at 1.83 eV with a relatively slower decay rate (orange and green lines). For the first feature, the large difference in ∆T between resonant and non-resonant excitation cannot be attributed to the energy difference in the pump pulses, since the photon energy of 3.10 eV is only 1.7 times as large as that of 1.83 eV. Even if we assume that the entire excitation energy of each photo-induced electron-hole pair is finally converted into heat within the MoS2 lattice, such an effect does not reconcile the observed large difference in ∆T (~30 times).

Figure 4. Two non-radiative energy dissipation channels after ultrafast excitation. (a) Comparing the temperature change ∆T in two samples under different excitation conditions, including MoS2 solution under excitation at 3.10 eV (180 µJ/cm2, red dots), MoS2 film under excitation at 1.83 eV (140 µJ/cm2, orange dots), and MoS2 solution under excitation at 1.83 eV (150 µJ/cm2, green dots). Colored lines show the corresponding bi-exponential fits to the data. (b) Schematic illustration of two distinctive non-radiative energy channels following ultrafast excitation. Two primary processes contribute to the lattice thermalization: (1) electron-phonon scattering in the inner layers during cooling of the hot electron-hole gas and the formation of bound carriers; and (2) the non-radiative recombination of the electron-hole pairs at


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the surface layers, during which (2’) most of the excess energy dissipates very rapidly from the MoS2 surfaces.

The most likely explanation is that the majority of the excess energy generated during carrier recombination dissipates very rapidly from MoS2 surfaces, which is different from that generated during the cooling of hot carriers. This is because defect-mediated non-radiative recombination dominates the decay channels in our samples and the fact that significantly more defect states are present at the surface layers, which act as recombination centers.38 As a consequence, most of the photo-induced electron-hole pairs combine at the surface layers of the MoS2 nanosheets, followed by a rapid dissipation of the excess energy, which perhaps occurs through a direct coupling to the low-frequency vibration modes of the surrounding solvent or underlying substrate. Therefore, the magnitude of the increase in lattice temperature is relatively small following resonant excitation, where carrier recombination is the principal thermalization pathway. In contrast, after non-resonant excitation with a photon energy far above the fundamental bandgap, lattice thermalization through electron-phonon scattering in the inner layers plays a dominant role. Furthermore, this conclusion also coincides with the different ∆T observed for the MoS2 nanosheets in solution and on a CaF2 substrate after excitation at 1.83 eV (orange and green lines in Figure 4a). Since the reduced temperature increase through carrier recombination arises from rapid heat dissipation on MoS2 surfaces, less contact of the surface layers with the surroundings in the CaF2 film can therefore lead to more


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residual heat left behind in the MoS2 lattice along with a slower lattice cooling rate. With these experimental results, we demonstrate the existence of two distinctive non-radiative energy channels in few-layer MoS2 following ultrafast excitation. As illustrated in Figure 4b, two primary processes contribute to the electronic energy dissipation. First, after ultrafast excitation with a pump photon energy larger than the fundamental bandgap, excess energy from the cooling of the generated hot electron-hole gas and the formation of bound carriers causes a fast thermalization of the MoS2 lattice via electron-phonon scattering in the inner layers. Then, the temperature of the lattice is further elevated by the non-radiative recombination of electron-hole pairs, which mainly occurs at the surface layers. However, the majority of the excess energy generated during carrier recombination dissipates very rapidly from the MoS2 surfaces through a direct coupling to the low-frequency vibration modes of the surroundings. CONCLUSIONS In summary, we carried out a comprehensive study for solution-processed few-layer MoS2 using ultrafast spectroscopy. By comparing experimental results obtained from pump-probe measurements under different excitation and detection conditions, the photo-induced carrier dynamics in few-layer MoS2 were clearly elaborated. As a consequence, the observed redshift of the exciton resonance energy within 10 to hundreds of picoseconds after photoexcitation is demonstrated to arise from the thermalization of the MoS2 lattice. Based on the linear relationship between the lattice temperature and the exciton resonance energy that extracted from the


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temperature-dependent absorption spectra for few-layer MoS2 in ethanol, the change in lattice temperature after ultrafast excitation was able to be directly monitored. Finally, two distinctive non-radiative energy channels following ultrafast excitation were revealed, including the cooling of hot carriers and the formation of bound carriers in the inner layers and the defect-mediated carrier recombination at the surface layers. Moreover, we present a useful method for the study of both electronic and phonon dynamics in TMDCs after ultrafast excitation and provide the necessary theoretical and experimental basis for the future design of TMDC-based optoelectronic devices as well as photocatalytic materials. MATERIALS AND METHODS Preparation of Few-layer MoS2 by Ion Exchange Reaction. Layered SnS2 was first prepared by liquid exfoliation.61 Then, few-layer SnS2 nanosheets (0.1 mmol), oleylamine (10 mmol), and 5 mL of 1-octadecene were loaded into a 50 mL three-necked flask. The mixture was degassed at 120 °C under a nitrogen atmosphere. Next, MoCl5 (2 mmol), oleic acid (3 mL) and trioctylphosphine (3 mL) were injected together into the precursor mixture. The reaction was maintained at a temperature of 320 °C for 5 hours. After the mixture cooled down to room temperature, the products were precipitated by adding ethanol and collected by centrifugation. More details for the process are described in the literature.42 The MoS2 film sample was prepared by evaporating the obtained solution onto a CaF2 window. Raman and AFM Characterization. Raman spectral measurements were carried out using a confocal micro-Raman spectrometer (WITec, alpha-300R). A 532-nm


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laser was used as the excitation source, which was focused onto the sample surface by using a ×100 objective lens. The scattered signals were collected by the same lens. AFM characterization was carried out with a NT-MDT Solver P47H-PRO (Russia NT-MDT Corporation) in tapping mode. Ultrafast Pump-Probe Spectroscopy. The spectroscopy setup utilized a femtosecond amplifier laser system (Spitfire Ace, Spectra Physics) that generated laser pulses with a repetition rate of 1 kHz, a central wavelength of 800 nm and a pulse duration of ~35 fs. The output was split into three beams. The first beam was used to pump an optical parametric amplifier (TOPAS, Spectra Physics) or frequency doubled to generate the 1.83 or 3.10 eV excitation pulses, respectively. The second beam with weaker energy was focused into a sapphire or an yttrium aluminum garnet plate to generate a white light continuum for the visible or near-infrared probe, respectively. The time delay between the pump beam and the probe beam was controlled by a motorized delay stage, with both beams focused onto the sample. After frequency resolved by a spectrograph, the excitation-induced transmission change for the probe light was collected by a home-built 46-channel synchronous digital lock-in amplifier,62 with optional Si photodiodes and InGaAs sensors for visible and near-infrared detection, respectively. For mid-infrared measurement, the above probe light was replaced by a third beam, which was directed to generate ultra-broadband super-continuum pulses that covered almost the whole mid-IR region by focusing fundamental light at 800 nm and the second harmonic at 400 nm simultaneously on air.63 The spectrum for the mid-infrared probe light was detected


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by a liquid-nitrogen-cooled mercury-cadmium-telluride array detector after frequency resolved by a spectrograph (iHR 320, HORIBA Jobin Yvon). Measurement of the Temperature-Dependent Absorption Spectra. A Hitachi U-3900 UV-Vis spectrophotometer was used to record the temperature-dependent absorption spectra. The MoS2 ethanol solution was placed in a quartz cuvette (4 mm optical length), with the temperature controlled from 11 °C to 79 °C by a water circulation system. The actual temperature was monitored in real time by an XMTF-8502 thermometer with an accuracy of 0.1 °C. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Raman and AFM characterization of the sample; calculated densities for the photo-induced electron-hole pairs; transient absorption spectra for the MoS2 film; fitting of the transient absorption spectrum with Lorentzian functions; time-dependent energy shift ∆E for the MoS2 B-exciton; details of multi-exponential fitting; measurements of excitation-induced absorption change for few-layer MoS2 at 0.25 eV; ultrafast dynamics after photoexcitation of the MoS2 film; temporal evolution of the energy shift ∆E and the change in lattice temperature ∆T for MoS2 nanosheets after resonant excitation. The authors declare no competing financial interest. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] ORCID Hailong Chen: 0000-0002-3456-7836 Zhuo Chen: 0000-0002-0671-4974 ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21603270, 21773302, 21633015 and 11721404), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDPB06). Z. Chen was financially supported by the National Natural Science Foundation of China (Grant No. 51472031).

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Table of Contents Graphic:


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Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2

Zhen Chi,†,§Huihui Chen,‡ Zhuo Chen,‡ Qing Zhao,§ † †,‖ Hailong Chen,*, and Yu-Xiang Weng

†

Beijing National Laboratory for Condensed Matter Physics, CAS Key Laboratory of

Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

Department of Materials Physics and Chemistry, Beijing Key Laboratory of

Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology Institution, Beijing 100081, China §

Center for Quantum Technology Research, School of Physics, Beijing Institute of

Technology, Beijing 10081, China ‖

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing

100049, China * Corresponding author: Hailong Chen (email: [email protected])


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ABSTRACT Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by non-radiative energy dissipation, has greatly limited practical applications. To reveal the details for non-radiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS2. With this method, the many-body effects, occupation effects and phonon dynamics are clearly identified. In particular, thermalization of the MoS2 lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within 10 to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (~9 ps) is mainly dissipated from the MoS2 surfaces to external phonons. KEYWORDS: 2D materials, ultrafast spectroscopy, electronic and phonon dynamics, energy dissipation, temperature sensor


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Two-dimensional transition metal dichalcogenides (TMDCs), such as MoS2, WS2, MoSe2, WSe2, etc., have gained tremendous attention in recent years due to their intriguing mechanical, electronic and optical properties.1-5 They offer great opportunities for fundamental and technological research in a variety of fields including

electronic

and

optoelectronic

devices,6-11

photocatalysis

and

electrocatalysis,12-18 energy storage,19-23 spin and valley physics,24-27 and so on. In all of these applications, non-radiative electronic energy dissipation in single- to few-layer TMDCs always plays a significant role. In particular, the relatively low quantum yield of current TMDC-based optoelectronic devices mainly originates from the non-radiative recombination of photo-induced carriers with the excess energy inevitably converted into heat,28 which can also lead to premature failure and shortening of the service life for the device. In general, two non-radiative energy channels should be carefully considered after photoexcitation of layered TMDCs (Figure 1a, curved arrows): (1) electron-phonon scattering during the ultrafast cooling of hot carriers and subsequent formation of bound carriers (e.g., excitons), and (2) non-radiative recombination of electron-hole pairs. A thorough study of these processes should be of central importance for revealing details of the electronic energy dissipation pathways in TMDCs. Ultrafast pump-probe spectroscopy has been widely used and demonstrated as a powerful tool for the study of various carrier dynamics in TMDCs, including carrier recombination,29-31 exciton-exciton annihilation,32,33 ultrafast charge transfer in TMDC heterostructures,34-36 spin-valley dynamics,26,37 etc. Despite such recent


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progress, detailed non-radiative energy dissipation dynamics in TMDCs following ultrafast photoexcitation remains poorly understood. One of the main reasons for this lack of understanding is that traditional optical pump-probe studies of single- to few-layer TMDCs are generally based on the measurement of resonant or near-resonant interband excitonic nonlinearities by tuning the probe wavelength close to the exciton line. The resulting data are usually difficult to interpret quantitatively,30 since, in most cases, there exists plenty of photo-induced dynamics (as mentioned above) that can be temporally mixed. To address this issue, we herein carry out a comprehensive ultrafast spectroscopy study for solution-processed few-layer MoS2 with the detection wavelength ranging from visible to near-infrared to mid-infrared. As schematically illustrated in Figure 1a, photo-induced unbound (or bound) carriers are first prepared by non-resonant (or resonant) excitation of femtosecond pump pulses. Then, the dynamics of various carriers are monitored by recording absorption spectra at different spectral regions with a controlled time delay between pump and probe pulses. In addition to traditional visible pump-probe spectroscopy, both near-infrared (NIR) and mid-infrared (MIR) pulses are also utilized in this work as optical probes. Based on the different response of photo-induced carriers on different probe photon energies in the near- to mid-infrared region, the dynamics of various kinds of bound and unbound carriers in few-layer MoS2 can be investigated.30,36,38-41 Specifically, the NIR spectrum is affected by the intraband absorption due to both free carriers and excitons,30 as well as the bleaching of optically active midgap defect states.38 In contrast, the MIR spectrum,


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if not close to the spectral range of intraexcitonic resonances,39,40 is only sensitive to free carriers and weakly bound carriers with binding energies well below the probe energy.36 Comparing all these results with those obtained from traditional visible pump-probe measurements, the photo-induced carrier dynamics in few-layer MoS2 are specifically described in detail. Most importantly, the underlying thermal effect after photoexcitation of MoS2 nanosheets is therefore clearly revealed. In particular, one can directly monitor the temporal evolution of lattice temperature. Further results demonstrate that the excess energy from the cooling of hot carriers and the formation of bound carriers plays a major role in the increase in lattice temperature, while the excess energy from defect-mediated carrier recombination mostly dissipates from the surface of MoS2 nanosheets during the recombination process. RESULTS AND DISCUSSION Photo-Induced Red- and Blueshift of the Exciton Resonance. Few-layer MoS2 nanosheets were prepared by ion exchange reaction in the solution phase,42 which is a low-cost and mass production method compared with mechanical exfoliation and chemical vapor deposition. For a better comparison, two samples containing MoS2 nanosheets either dissolved in ethanol or coated onto a calcium fluoride (CaF2) window were measured in this work. The dominated thickness of the nanosheets is approximately 6-7 layers, which was confirmed by both Raman spectroscopy and atomic force microscopy (see Supporting Information Figure S1). Except for the measurement of temperature-dependent absorption spectra, all the experiments were carried out at room temperature and under ambient conditions. The absorption spectra


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for the two samples show almost the same features (Figure 1b), indicating the electronic structure of MoS2 was barely altered during the coating process. Both spectra show characteristic A- and B-exciton resonances of approximately 1.83 eV and 1.98 eV, respectively, which are ascribed to excitonic transitions occurring at the K/K' points of the k-space.43,44

Figure 1. Schematic diagram of the multiple relaxation pathways for the photo-induced carriers and the observed ultrafast spectra in the visible region of the few-layer MoS2 nanosheets. (a) Schematic of the ultrafast pump-probe spectroscopy with optional visible (Vis), near-infrared (NIR) and mid-infrared (MIR) detection irradiation. Dashed and solid straight arrows denote possible optical transitions caused by the pump and probe pulses, respectively. Solid and dashed parabolas schematically show the dispersion states with a principal quantum number n = 1, 2, and so forth and the carrier continuum state. Curved arrows indicate two major electronic energy dissipation processes, as discussed in the main text. (b) Absorption spectra for few-layer MoS2 nanosheets in ethanol (solution) and on a calcium fluoride substrate (film). Red and blue shaded areas indicate the pump photon energies for resonant and non-resonant excitations, respectively. (c) Transient absorption spectra for the sample in ethanol at selected time delays following photoexcitation at 1.83 eV (left, red lines) and 3.10 eV (right, blue lines) with a pump fluence of 64 and 77 μJ/cm2, respectively. Absorption spectrum at a time delay of -0.4 ps representing the response prior to excitation is included in each panel for


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comparison (gray lines). The broad background in each transient absorption spectrum has been removed.

Visible pump-probe measurements, which focus on the optical response at the Aand B-exciton resonances after photoexcitation, were first carried out for the two different samples. The photon energy of the pump pulses was tuned to be 1.83 eV for resonant excitation and 3.10 eV for non-resonant excitation (shaded areas in Figure 1b), and the pump fluence varied from 37 to 150 μJ/cm2 and 44 to 180 μJ/cm2, respectively. The excitation at 1.83 eV, which is close to the MoS2 A-exciton absorption maximum, creates bound carriers (e.g., excitons and weakly bound electron/hole pairs36), while that at 3.10 eV, which is far above the fundamental bandgap, predominantly induces free charge carriers.36,39 Figure 1c shows the transient absorption spectra for MoS2 in ethanol at different time delays following photoexcitation at 1.83 and 3.10 eV. The absorption spectrum measured at a time delay of -0.4 ps representing the response prior to excitation is also plotted in each panel for comparison (gray lines). The broad background in each transient absorption spectrum was removed by fitting with a quadratic polynomial. The pump pulses with a fluence of 64 μJ/cm2 for excitation at 1.83 eV and 77 μJ/cm2 for excitation at 3.10 eV generate electron-hole pairs with almost equal density per MoS2 layer (~8.51012 cm-2, see Supporting Note 1), below the Mott threshold for the MoS2 nanosheets.45,46 All the transient absorption spectra exhibit unambiguous bleaching of the exciton resonances following ultrafast excitation, which fully recover


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after hundreds of picoseconds. Interestingly, a blueshift of the exciton resonance energy upon excitation at 1.83 eV is clearly observed (Figure 1c, left panel), while a pronounced redshift appears when the pump photon energy is turned to 3.10 eV (Figure 1c, right panel). The same phenomenon was also observed in measurements of MoS2 nanosheets on a CaF2 substrate (see Supporting Information Figure S2). To clearly identify the time-dependent exciton resonance energy shift, a line shape analysis was carried out by fitting all the measured transient absorption spectra with Lorentzian functions (see Supporting Information Figure S3). Three fitted peaks are identified that are attributed to the trion (A-, ~1.79 eV), A-exciton (~1.83 eV) and B-exciton (~1.97 eV).47,48 Figure 2a shows the time-dependent energy shift ΔE for the MoS2 A-exciton in ethanol after resonant (1.83 eV) and non-resonant (3.10 eV) excitation with different pump fluences. Following non-resonant excitation, a negative ΔE corresponding to a redshift of the resonance energy immediately appears, the value of which increases significantly at higher pump fluence. Even after tens of picoseconds, a notable redshift of the exciton resonance energy is still observed. In contrast, after resonant excitation, a positive ΔE (blueshift) reaches its maximum immediately, followed by a fast decay within several picoseconds. Only a slight redshift can be observed at tens of picoseconds, which becomes more obvious at higher pump fluence. The same behavior was also observed for the time-dependent energy shift for the B-exciton (see Supporting Information Figure S4).

Figure 2. Ultrafast dynamics after photoexcitation of MoS2 nanosheets in ethanol


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measured at different probe photon energies. (a) Time-dependent energy shift ΔE for the MoS2 A-exciton after excitation at 1.83 eV and 3.10 eV with different pump fluences. (b) Temporal evolution of the excitation-induced transmission change for the MoS2 nanosheets detected at 1.31 eV under excitation at 1.83 eV (140 μJ/cm2). (c) Temporal evolution of the excitation-induced absorption change for the MoS2 nanosheets detected at 0.25 eV under excitation at 1.83 eV (110 μJ/cm2) and 3.10 eV (110 μJ/cm2). For (a)-(c), the dots are the data, and the curves show multi-exponential fitting that includes consideration of the instrument response function (~150 fs). (d) Schematic illustration of the overall scenario after photoexcitation of few-layer MoS2 for which the dynamic process can be described roughly in three stages.

We first discuss the opposite energy shifts observed at the beginning after resonant and non-resonant excitation. As previously reported, the presence of photo-induced carriers in MoS2 leads to Pauli-blocking of the occupied states, which results in bandgap renormalization and plasma screening of the Coulomb interaction.49 The former effect usually causes a redshift of the exciton resonance energy due to band gap shrinkage, with the latter leading to a blueshift that can be attributed to a decrease in the exciton binding energy.46 Therefore, the overall shift in the resonance energy can be ascribed to a competition between band gap shrinkage and decreasing exciton binding energy.49-51 Our experimental results indicate that the former is relatively more significant in few-layer MoS2 under non-resonant excitation, where an abundance of free charge carriers are generated. In contrast, the reduced exciton


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binding energy plays a dominant role after resonant excitation, which mainly leads to the creation of bound electron-hole pairs (e.g., excitons). However, the observed redshift at tens of picoseconds for both the resonant and non-resonant excitations cannot be explained by the competition between these two effects. As discussed in the following sections, this redshift arises from the elevation of the lattice temperature following photoexcitation, which shifts the exciton peak position to lower energies. Electronic and Phonon Dynamics after Ultrafast Excitation. To clarify the various carrier dynamics in few-layer MoS2 and to therefore extract the underlying thermal effect after ultrafast excitation, the time scales for the different dynamic processes should be first clearly identified. As shown in Figure 2a, the temporal evolution of ΔE after 3.10 eV excitation can be described roughly in three stages that are marked with different colors: (I) a fast decrease and recovery of ΔE within 2 ps, (II) a following recovery for ΔE, with a time constant of approximately 15 ps, and (III) finally, a slow recovery of ΔE that lasts for more than hundreds of picoseconds. All three time scales show no significant change with increasing pump fluence. Similarly, after 1.83 eV excitation, the blueshift of the exciton resonance rapidly reaches its maximum, followed by a fast (~2 ps, stage I) and slow decay (~8 ps, stage II). Then, the ΔE turns negative and lasts for hundreds of picoseconds (stage III). All the time scales are also found to be independent of the pump fluence. To explicitly assign these processes, we further successively tuned the probe photon energy to 1.31 eV (NIR) and 0.25 eV (MIR). As shown in Figure 2b, the temporal evolution for the excitation-induced transmission change at 1.31 eV shows two bleaching recovery


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components, with time constants of 2 ps (~80％, stage I) and 9 ps (~20％, stage II). Fitting details are provided in Supporting Note 2. By comparison, a rapid increase in the transient absorption signal after photoexcitation is observed at the MIR region (Figure 2c), followed by a fast decay within 2 ps (stage I) for both resonant and non-resonant excitation. As mentioned above, the MIR detection signal is predominantly due to the absorption of free charge carriers as well as weakly bound electron/hole pairs. Since the photon energy of 0.25 eV is well below the excitonic-binding energy for the layered MoS2, the tightly bound carriers, such as excitons and trapped carriers, can barely absorb the probe photons and act as a charge-neutral insulating gas.30,52 Therefore, the signal decays shown in Figure 2c mainly reflect the rapid conversion of photo-induced free carriers (3.10 eV excitation) and weakly-bound electron/hole pairs (1.83 eV excitation) into tightly bound carriers.36 Multi-exponential fitting of the data reveals two decay constants of approximately 0.5 ps (64％) and 3 ps (36％) for both resonant and non-resonant excitation, which are almost independent of the pump fluence (see Supporting Information Figure S5 and Table S1). The faster decay component can be attributed to the ultrafast formation of tightly bound excitons,36 which is consistent with previous observations for a WSe2 monolayer.39 Decay dynamics with a time constant of ~3 ps were also observed in previous studies for monolayer and few-layer MoS2, which was ascribed to a fast trapping of carriers by surface trap states.29,30 In addition, compared with excitation at 1.83 eV excitation, an extra exponential rise with a time constant of 90 fs is clearly observed under


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excitation at 3.10 eV (see Supporting Information Figure S6 and Table S1). This reflects the initial relaxation process for the non-resonant excitation-induced hot electron-hole gas via strong electron-electron scattering.53,54 As a consequence, most of the photo-induced carriers form tightly bound excitons and trapped carriers within 2 ps (stage I). Furthermore, the bleaching signal observed in the NIR region (Figure 2b) reflects the subsequent dynamics. In a previous study,38 a weak absorption band in the 0.8-1.6 eV range was observed in few-layer MoS2, which is due to optical transitions from midgap defect states to the conduction bands. The positive ΔT/T observed immediately after photoexcitation was attributed to decreased defect absorption by the probe pulse due to ionization of the defects by photoexcitation.38 Therefore, the slow decay component on the picosecond time scale (~9 ps) in Figure 2b could be assigned to non-radiative recombination of trapped electron-hole pairs (stage II). So far, the temporal evolutions for the exciton resonance shift ΔE shown in Figure 2a can be clearly described. The observed fast recovery component (~2 ps) for excitation at both 1.83 and 3.10 eV mainly arises from the formation of tightly bound excitons and trapped carriers following ultrafast excitation, and the relatively slow component (~8 ps) after excitation at 1.83 eV arises from non-radiative carrier recombination. As widely reported, defect-mediated non-radiative recombination (Shockley-Read-Hall recombination) and Auger-type biexcitonic recombination at higher excitation powers dominate the decay channels for layered MoS2.30,32,38,55-57 In our case, the former recombination mechanism plays a dominant role, since all the


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time constants discussed above are nearly independent of the pump fluence. However, the ~15 ps recovery component for the negative ΔE in stage II for excitation at 3.10 eV (Figure 2a) cannot be simply attributed to the non-radiative recombination process, since the dominant bound electron-hole pairs existing in this stage should result in a positive ΔE, as discussed above. To explain this issue, the thermal effect should be carefully considered here, which has been previously demonstrated to play a particularly important role in pump-probe experiments.51,58 Specifically, with the formation of bound carriers from a non-resonant excitation-induced hot electron-hole gas, the excess energy is inevitably dissipated into the lattice via electron-phonon scattering. The subsequent increase in the local temperature will shift the exciton resonance to lower energies, in accordance with the temperature-dependent shift observed for the band gap. To verify this effect, temperature-dependent absorption spectra were collected for MoS2 nanosheets in ethanol. As shown in Figure 3a, the resonance energies for both the A-exciton and B-exciton monotonically decrease with increasing temperature, which is consistent with the above conclusion. Therefore, the heating of the lattice by the hot electron-hole gas via electron-phonon coupling is responsible for the negative ΔE observed in stage II under excitation at 3.10 eV (Figure 2a), and the slow recovery that lasts hundreds of picoseconds in stage III represents the cooling process for the lattice following ultrafast thermalization.

Figure 3. Temporal evolution for the lattice temperature in MoS2 nanosheets after thermalization. (a) Temperature-dependent absorption spectra for few-layer


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MoS2 in ethanol. The broad background in each absorption spectrum has been removed. (b) The exciton resonance energies extracted from (a) are plotted as a function of temperature. (c) & (d) Temporal evolution of the lattice temperature change ΔT in MoS2 nanosheets after excitation at 3.10 eV with pump varying fluence. The data (dots) along with bi-exponential fitting (lines) are extracted from the energy shift of the A-exciton resonance after 5 ps for the MoS2 solution (c) and film (d). Insets of (c) and (d) plot the corresponding temperature increase T under varying pump fluence for a time delay of 10 ps, with all the data (dots) fitted to a linear function (lines).

The same experimental results were obtained for MoS2 nanosheets on a CaF2 substrate (see Supporting Information Figure S7, Figure S8 and Table S2). Based on the above analysis, the time scales of different dynamic processes after ultrafast excitation are clearly identified. For example, the many-body effect such as electron-electron scattering is observed within 100 fs after photoexcitation by measuring the MIR detection signal, and the electron-phonon scattering is demonstrated to occur on the picosecond time scale, which is responsible for the observed redshift of the exciton resonance energy on longer time scales. After photoexcitation of few-layer MoS2, the overall scenario can be roughly described by three stages with different corresponding time scales, as schematically depicted in Figure 2d. Temporal Evolution of the Lattice Temperature after Ultrafast Thermalization.


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From the temperature-dependent absorption spectra for few-layer MoS2 in ethanol (Figure 3a), the exciton resonance energies at different temperatures can be extracted by fitting with Lorentzian functions (see Supporting Information Figure S3). As shown in Figure 3b, the resonance energies for both the A-exciton and B-exciton decrease linearly with increasing temperature from 284 to 352 K. From linear fitting, the temperature coefficients for the A-exciton and B-exciton are obtained as -0.42 and -0.49 meV/K, respectively, which are very close to each other. Similar values lying between -0.26 and -0.5 meV/K have been extracted for various TMDCs from temperature-dependent photoluminescence measurements.59,60 Based on the linear relationship between the temperature and resonance energy for the A-exciton, the excitation-induced temperature increase, ΔT, for MoS2 can be well estimated.51 The temporal evolution for ΔT in MoS2 solution and film after excitation at 3.10 eV with varying pump fluence is shown in Figure 3c and 3d, which was extracted from the corresponding time-dependent exciton resonance shift. Only the data measured after a time delay of 5 ps are shown here, for which the thermal effect dominates the redshift of the resonance energies. The insets of Figure 3c and 3d plot the temperature increases T under varying pump fluence 10 ps after excitation, when most of the electron-hole pairs have recombined. The lattice temperature increase shows a linear relationship with the pump fluence for both the MoS2 solution and film, which is consistent with a previous report.51 The slight deviation in ΔT from a linear relationship for the MoS2 film at 180 μJ/cm2 is probably because the density of the generated electron-hole pairs is too close to the Mott threshold for the MoS2


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nanosheets. All these data for the temporal evolution of ΔT exhibit a bi-exponential decay feature. Table 1 lists the fitted time constants for the decay curves shown in Figure 3c and 3d. The faster decay component can be attributed to a rapid equilibrium of the lattice temperature within the entire nanosheets after ultrafast local thermalization, with the relatively slower component corresponding to the transfer of heat from the MoS2 nanosheets to the surrounding solvent (solution) or underlying substrate (film). The equilibrium time for the lattice temperature for the MoS2 in ethanol (~15 ps) is clearly longer than that for the MoS2 on the CaF2 substrate (~5 ps). This is probably because the MoS2 nanosheets in ethanol are more flexible, retarding the heat dissipation within the entire nanosheets. In contrast, the final lattice cooling time for the MoS2 film (~1100 ps) is much longer than that in ethanol (~550 ps). This could be explained by the different environment of the MoS2 nanosheets in the two samples. The MoS2 nanosheets in ethanol are completely surrounded by solvent molecules to which generated heat can be transferred with high efficiency. For MoS2 nanosheets on the CaF2 substrate, more than half of the MoS2 surfaces are in contact with air or other MoS2 nanosheets, limiting heat transfer efficiency to the CaF2 substrate. These results also confirm that the temporal evolution of the lattice temperature in MoS2 nanosheets after thermalization can be well monitored by using this method.

Table 1. Fitted time constants 1 and 2 for the decay curves (ΔT) shown in Figure 3c and 3d.


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Pump Fluence

solution

film

1 (ps)

2 (ps)

1 (ps)

2 (ps)

44 μJ/cm2

19±2

550±10

4.7±0.2

1100±30

55 μJ/cm2

16±1

520±10

4.7±0.1

1100±20

77 μJ/cm2

16±1

560±10

5.5±0.2

1080±20

110 μJ/cm2

14±1

550±10

6.3±0.2

1060±20

180 μJ/cm2

13±1

520±10

7.1±0.2

1510±30

Rapid Energy Dissipation through Defect-Mediated Carrier Recombination. The temporal evolution of the energy shift E for the MoS2 solution and film excited at 1.83 eV is shown in Supporting Information Figure S9. Similar to the data shown in Figure 2a, a redshift following a blueshift is observed at a time delay of approximately 15 ps and 10 ps for the solution and film, respectively. Furthermore, the redshift becomes more significant with increasing pump fluence, especially for the sample on the CaF2 substrate. It is well known that non-radiative recombination of electron-hole pairs always generates heat in materials via activation of the phonons in the lattice. Thus, the redshift observed at a relatively longer time delay after resonant excitation can be attributed to a thermal effect that is mainly caused by non-radiative recombination. In Figure 4a, the temporal evolution of ΔT is compared for the MoS2 solution and the film under different excitation conditions, with the initial density of photo-induced electron-hole pairs per layer nearly equal in both cases (~1.91013 cm-2, see Supporting Note 1). Two interesting features are identified: (1) the maximum


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temperature increase ΔT for the sample in solution after photoexcitation at 3.10 eV is almost 30 times larger than the ΔT observed following excitation at 1.83 eV (red and green lines), and (2) compared with the data measured in solution, the MoS2 on the CaF2 substrate can reach a higher temperature after excitation at 1.83 eV with a relatively slower decay rate (orange and green lines). For the first feature, the large difference in ΔT between resonant and non-resonant excitation cannot be attributed to the energy difference in the pump pulses, since the photon energy of 3.10 eV is only 1.7 times as large as that of 1.83 eV. Even if we assume that the entire excitation energy of each photo-induced electron-hole pair is finally converted into heat within the MoS2 lattice, such an effect does not reconcile the observed large difference in ΔT (~30 times).

Figure 4. Two non-radiative energy dissipation channels after ultrafast excitation. (a) Comparing the temperature change ΔT in two samples under different excitation conditions, including MoS2 solution under excitation at 3.10 eV (180 μJ/cm2, red dots), MoS2 film under excitation at 1.83 eV (140 μJ/cm2, orange dots), and MoS2 solution under excitation at 1.83 eV (150 μJ/cm2, green dots). Colored lines show the corresponding bi-exponential fits to the data. (b) Schematic illustration of two distinctive non-radiative energy channels following ultrafast excitation. Two primary processes contribute to the lattice thermalization: (1) electron-phonon scattering in the inner layers during cooling of the hot electron-hole gas and the formation of bound carriers; and (2) the non-radiative recombination of the electron-hole pairs at


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the surface layers, during which (2’) most of the excess energy dissipates very rapidly from the MoS2 surfaces.

The most likely explanation is that the majority of the excess energy generated during carrier recombination dissipates very rapidly from MoS2 surfaces, which is different from that generated during the cooling of hot carriers. This is because defect-mediated non-radiative recombination dominates the decay channels in our samples and the fact that significantly more defect states are present at the surface layers, which act as recombination centers.38 As a consequence, most of the photo-induced electron-hole pairs combine at the surface layers of the MoS2 nanosheets, followed by a rapid dissipation of the excess energy, which perhaps occurs through a direct coupling to the low-frequency vibration modes of the surrounding solvent or underlying substrate. Therefore, the magnitude of the increase in lattice temperature is relatively small following resonant excitation, where carrier recombination is the principal thermalization pathway. In contrast, after non-resonant excitation with a photon energy far above the fundamental bandgap, lattice thermalization through electron-phonon scattering in the inner layers plays a dominant role. Furthermore, this conclusion also coincides with the different T observed for the MoS2 nanosheets in solution and on a CaF2 substrate after excitation at 1.83 eV (orange and green lines in Figure 4a). Since the reduced temperature increase through carrier recombination arises from rapid heat dissipation on MoS2 surfaces, less contact of the surface layers with the surroundings in the CaF2 film can therefore lead to more


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residual heat left behind in the MoS2 lattice along with a slower lattice cooling rate. With these experimental results, we demonstrate the existence of two distinctive non-radiative energy channels in few-layer MoS2 following ultrafast excitation. As illustrated in Figure 4b, two primary processes contribute to the electronic energy dissipation. First, after ultrafast excitation with a pump photon energy larger than the fundamental bandgap, excess energy from the cooling of the generated hot electron-hole gas and the formation of bound carriers causes a fast thermalization of the MoS2 lattice via electron-phonon scattering in the inner layers. Then, the temperature of the lattice is further elevated by the non-radiative recombination of electron-hole pairs, which mainly occurs at the surface layers. However, the majority of the excess energy generated during carrier recombination dissipates very rapidly from the MoS2 surfaces through a direct coupling to the low-frequency vibration modes of the surroundings. CONCLUSIONS In summary, we carried out a comprehensive study for solution-processed few-layer MoS2 using ultrafast spectroscopy. By comparing experimental results obtained from pump-probe measurements under different excitation and detection conditions, the photo-induced carrier dynamics in few-layer MoS2 were clearly elaborated. As a consequence, the observed redshift of the exciton resonance energy within 10 to hundreds of picoseconds after photoexcitation is demonstrated to arise from the thermalization of the MoS2 lattice. Based on the linear relationship between the lattice temperature and the exciton resonance energy that extracted from the


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temperature-dependent absorption spectra for few-layer MoS2 in ethanol, the change in lattice temperature after ultrafast excitation was able to be directly monitored. Finally, two distinctive non-radiative energy channels following ultrafast excitation were revealed, including the cooling of hot carriers and the formation of bound carriers in the inner layers and the defect-mediated carrier recombination at the surface layers. Moreover, we present a useful method for the study of both electronic and phonon dynamics in TMDCs after ultrafast excitation and provide the necessary theoretical and experimental basis for the future design of TMDC-based optoelectronic devices as well as photocatalytic materials. MATERIALS AND METHODS Preparation of Few-layer MoS2 by Ion Exchange Reaction. Layered SnS2 was first prepared by liquid exfoliation.61 Then, few-layer SnS2 nanosheets (0.1 mmol), oleylamine (10 mmol), and 5 mL of 1-octadecene were loaded into a 50 mL three-necked flask. The mixture was degassed at 120 °C under a nitrogen atmosphere. Next, MoCl5 (2 mmol), oleic acid (3 mL) and trioctylphosphine (3 mL) were injected together into the precursor mixture. The reaction was maintained at a temperature of 320 °C for 5 hours. After the mixture cooled down to room temperature, the products were precipitated by adding ethanol and collected by centrifugation. More details for the process are described in the literature.42 The MoS2 film sample was prepared by evaporating the obtained solution onto a CaF2 window. Raman and AFM Characterization. Raman spectral measurements were carried out using a confocal micro-Raman spectrometer (WITec, alpha-300R). A 532-nm


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laser was used as the excitation source, which was focused onto the sample surface by using a 100 objective lens. The scattered signals were collected by the same lens. AFM characterization was carried out with a NT-MDT Solver P47H-PRO (Russia NT-MDT Corporation) in tapping mode. Ultrafast Pump-Probe Spectroscopy. The spectroscopy setup utilized a femtosecond amplifier laser system (Spitfire Ace, Spectra Physics) that generated laser pulses with a repetition rate of 1 kHz, a central wavelength of 800 nm and a pulse duration of ~35 fs. The output was split into three beams. The first beam was used to pump an optical parametric amplifier (TOPAS, Spectra Physics) or frequency doubled to generate the 1.83 or 3.10 eV excitation pulses, respectively. The second beam with weaker energy was focused into a sapphire or an yttrium aluminum garnet plate to generate a white light continuum for the visible or near-infrared probe, respectively. The time delay between the pump beam and the probe beam was controlled by a motorized delay stage, with both beams focused onto the sample. After frequency resolved by a spectrograph, the excitation-induced transmission change for the probe light was collected by a home-built 46-channel synchronous digital lock-in amplifier,62 with optional Si photodiodes and InGaAs sensors for visible and near-infrared detection, respectively. For mid-infrared measurement, the above probe light was replaced by a third beam, which was directed to generate ultra-broadband super-continuum pulses that covered almost the whole mid-IR region by focusing fundamental light at 800 nm and the second harmonic at 400 nm simultaneously on air.63 The spectrum for the mid-infrared probe light was detected


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by a liquid-nitrogen-cooled mercury-cadmium-telluride array detector after frequency resolved by a spectrograph (iHR 320, HORIBA Jobin Yvon). Measurement of the Temperature-Dependent Absorption Spectra. A Hitachi U-3900 UV-Vis spectrophotometer was used to record the temperature-dependent absorption spectra. The MoS2 ethanol solution was placed in a quartz cuvette (4 mm optical length), with the temperature controlled from 11 °C to 79 °C by a water circulation system. The actual temperature was monitored in real time by an XMTF-8502 thermometer with an accuracy of 0.1 °C. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Raman and AFM characterization of the sample; calculated densities for the photo-induced electron-hole pairs; transient absorption spectra for the MoS2 film; fitting of the transient absorption spectrum with Lorentzian functions; time-dependent energy shift ΔE for the MoS2 B-exciton; details of multi-exponential fitting; measurements of excitation-induced absorption change for few-layer MoS2 at 0.25 eV; ultrafast dynamics after photoexcitation of the MoS2 film; temporal evolution of the energy shift ΔE and the change in lattice temperature ΔT for MoS2 nanosheets after resonant excitation. The authors declare no competing financial interest. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] ORCID Hailong Chen: 0000-0002-3456-7836 Zhuo Chen: 0000-0002-0671-4974 ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21603270, 21773302, 21633015 and 11721404), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDPB06). Z. Chen was financially supported by the National Natural Science Foundation of China (Grant No. 51472031).

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Table of Contents Graphic:


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Figure 1. Schematic diagram of the multiple relaxation pathways for the photo-induced carriers and the observed ultrafast spectra in the visible region of the few-layer MoS2 nanosheets. (a) Schematic of the ultrafast pump-probe spectroscopy with optional visible (Vis), near-infrared (NIR) and mid-infrared (MIR) detection irradiation. Dashed and solid straight arrows denote possible optical transitions caused by the pump and probe pulses, respectively. Solid and dashed parabolas schematically show the dispersion states with a principal quantum number n = 1, 2, and so forth and the carrier continuum state. Curved arrows indicate two major electronic energy dissipation processes, as discussed in the main text. (b) Absorption spectra for few-layer MoS2 nanosheets in ethanol (solution) and on a calcium fluoride substrate (film). Red and blue shaded areas indicate the pump photon energies for resonant and non-resonant excitations, respectively. (c) Transient absorption spectra for the sample in ethanol at selected time delays following photoexcitation at 1.83 eV (left, red lines) and 3.10 eV (right, blue lines) with a pump fluence of 64 and 77 µJ/cm2, respectively. Absorption spectrum at a time delay of -0.4 ps representing the response prior to excitation is included in each panel for comparison (gray lines). The broad background in each transient absorption spectrum has been removed. 108x73mm (300 x 300 DPI)


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Figure 2. Ultrafast dynamics after photoexcitation of MoS2 nanosheets in ethanol measured at different probe photon energies. (a) Time-dependent energy shift ∆E for the MoS2 A-exciton after excitation at 1.83 eV and 3.10 eV with different pump fluences. (b) Temporal evolution of the excitation-induced transmission change for the MoS2 nanosheets detected at 1.31 eV under excitation at 1.83 eV (140 µJ/cm2). (c) Temporal evolution of the excitation-induced absorption change for the MoS2 nanosheets detected at 0.25 eV under excitation at 1.83 eV (110 µJ/cm2) and 3.10 eV (110 µJ/cm2). For (a)-(c), the dots are the data, and the curves show multi-exponential fitting that includes consideration of the instrument response function (~150 fs). (d) Schematic illustration of the overall scenario after photoexcitation of few-layer MoS2 for which the dynamic process can be described roughly in three stages. 119x168mm (300 x 300 DPI)


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Figure 3. Temporal evolution for the lattice temperature in MoS2 nanosheets after thermalization. (a) Temperature-dependent absorption spectra for few-layer MoS2 in ethanol. The broad background in each absorption spectrum has been removed. (b) The exciton resonance energies extracted from (a) are plotted as a function of temperature. (c) & (d) Temporal evolution of the lattice temperature change ∆T in MoS2 nanosheets after excitation at 3.10 eV with pump varying fluence. The data (dots) along with bi-exponential fitting (lines) are extracted from the energy shift of the A-exciton resonance after 5 ps for the MoS2 solution (c) and film (d). Insets of (c) and (d) plot the corresponding temperature increase ∆T under varying pump fluence for a time delay of 10 ps, with all the data (dots) fitted to a linear function (lines). 134x113mm (300 x 300 DPI)


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Figure 4. Two non-radiative energy dissipation channels after ultrafast excitation. (a) Comparing the temperature change ∆T in two samples under different excitation conditions, including MoS2 solution under excitation at 3.10 eV (180 µJ/cm2, red dots), MoS2 film under excitation at 1.83 eV (140 µJ/cm2, orange dots), and MoS2 solution under excitation at 1.83 eV (150 µJ/cm2, green dots). Colored lines show the corresponding bi-exponential fits to the data. (b) Schematic illustration of two distinctive non-radiative energy channels following ultrafast excitation. Two primary processes contribute to the lattice thermalization: (1) electron-phonon scattering in the inner layers during cooling of the hot electron-hole gas and the formation of bound carriers; and (2) the non-radiative recombination of the electron-hole pairs at the surface layers, during which (2’) most of the excess energy dissipates very rapidly from the MoS2 surfaces. 54x18mm (300 x 300 DPI)


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Ultrafast Energy Dissipation via Coupling with Internal and External

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