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Charge Trapping and Exciton Dynamics in Large-area CVD Grown MoS Paul D. Cunningham, Kathleen Michelle McCreary, Aubrey T Hanbicki, Marc Currie, Berend T. Jonker, and L. Michael Hayden J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00647 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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The Journal of Physical Chemistry

Charge Trapping and Exciton Dynamics in Large-‐area CVD Grown MoS2 Paul D. Cunningham,1* Kathleen M. McCreary,1 Aubrey T. Hanbicki,1 Marc Currie,1 Berend T. Jonker,1 and L. Michael Hayden2,† 1U.S. Naval Research Laboratory, Washington, DC 20375 2Department of Physics, UMBC, Baltimore, MD 21250 *[email protected] †[email protected]

Abstract There is keen interest in monolayer transition metal dichalcogenide films for a variety of optoelectronic applications due to their direct band-‐gap and fast carrier dynamics. However, the mechanisms dominating their carrier dynamics are poorly understood. By combining time-‐resolved terahertz (THz) spectroscopy and transient absorption, we are able to shed light on the optoelectronic properties of large area CVD grown mono-‐ and multi-‐layer MoS2 films and determine the origins of the characteristic two-‐component excited state dynamics. The photo-‐induced conductivity shows that charge carriers, and not excitons, are responsible for the sub-‐picosecond dynamics. Identical dynamics resulting from sub-‐optical gap excitation suggest that charge carriers are rapidly trapped by mid-‐gap states within 600 fs. This process complicates the excited state spectrum with rapid changes in line-‐width broadening in addition to a red-‐shift due to band gap renormalization and simple state-‐filling effects. These dynamics are insensitive to film thickness, temperature, or choice of substrate, which suggests that carrier trapping occurs at surface defects or grain boundaries. The slow dynamics are associated with exciton recombination, and lengthen from 50 ps for monolayer films to 150 ps for multi-‐ layer films indicating that surface recombination dominates their lifetime. We see no signatures of trions in these MoS2 films. Our results imply that CVD grown films of MoS2 hold potential for high-‐speed optoelectronics and provide an explanation for the absence of trions in some CVD grown MoS2 films. Introduction Two-‐dimensional (2D) transition metal dichalcogenides (TMD) are of interest for optoelectronic applications due to their direct band-‐gaps and strong photoluminescence exhibited in single-‐layers.1 Similar to graphene, strong in-‐plane bonds and weak van der Waals forces between atomic layers allow single layer TMDs to be isolated and potentially reassembled monolayer by monolayer into heterostructures.2 Of the TMDs, MoS2 was the first in which the now familiar transition from bulk indirect band-‐gap to monolayer direct band-‐gap was observed.3-‐4 Novel electronic and photonic devices have been demonstrated using MoS2 including high mobility GHz-‐frequency field-‐effect transistors with large on/off ratios,5 fast response phototransistors that can be sensitized into the infrared,6 and extremely sensitive chemical sensors.7 Spin control of the K and K’ valley populations has

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also led to intense interest in MoS2 for valleytronics applications.8 Though much of the work has focused on exfoliated flakes, other deposition methods including chemical vapor deposition (CVD)9 have been pursued to produce large area monolayer films.10-‐12 Understanding the photo-‐physical properties of large area CVD grown MoS2 films is crucial to advancing many of the aforementioned optoelectronic applications. Previous photo-‐physical studies of both monolayer and multilayer MoS2 have observed complex multiexponential dynamics.13-‐14 The interpretation of these results is heavily debated and the underlying physics has been assigned to a number of different phenomena. Fast, picosecond scale dynamics have been assigned to the electron-‐hole (exciton) radiative lifetime,15 charge carrier trapping,16 charge carrier cooling,17 trion formation,18 and coupling between the A and B excitons.14 Slower decays >10 picoseconds have been assigned to interband recombination,13 exciton-‐phonon scattering,19 and the lifetime of charged excitons (e.g. trions).20 Clearly, the excited state photo-‐physics in MoS2 is not yet well understood and requires greater clarification for a consensus to be reached. Of these prior reports, thorough studies of the excited state photo-‐physics using transient absorption (TA) techniques have focused on multilayer MoS2,14, 17-‐18 leaving technologically important monolayers relatively unexplored. Nearly all of the previous photo-‐physical studies have relied on traditional all-‐ optical methods, such as photoluminescence and optical pump-‐probe techniques, to probe the excited state dynamics. These methods cannot directly probe charge conduction. However, time-‐resolved THz spectroscopy (TRTS) provides unique insight into excited state photo-‐physics, with its ability to discern between charge carriers and excitons with sub-‐picosecond resolution.21 Very recently, two TRTS studies of CVD grown monolayer MoS2 reported dramatically different observations. Lui et al. report a ~30 ps lifetime photo-‐ induced decrease in conductivity due to charged exciton (e.g. trion) formation.20 Docherty et al. report a sub-‐picosecond lifetime photo-‐induced increase in conductivity from short-‐ lived charge carriers.22 The source of these short-‐lived dynamics, which are similar to what has been previously observed with more traditional optical spectroscopy,14-‐15 remains the subject of debate. Here we utilize both ultrafast TA spectroscopy and TRTS to determine the origins of the fast charge carrier dynamics in mono-‐ and multi-‐layer films of MoS2 on sapphire, fused silica, and cyclic olefin co-‐polymer (TOPAS®) substrates. TA reveals two decay components associated with different photo-‐physical processes. The fast component is accompanied by changes in line width, associated with a rapid reduction in excited states, and a small spectral shift due to band-‐gap renormalization. Using TRTS, we observe a photo-‐induced increase in conductivity with identical dynamics, which confirms that mobile charge carriers, and not excitons, produce the fast decay. These charge carriers are trapped within approximately 600 fs. The independence of this decay on substrate or film thickness suggests that carriers are trapped at either surface defects or grain boundaries. The observation of photocurrent for photon energies below the optical gap points to the presence of mid-‐gap states associated with these traps. Weak temperature, fluence, and photon-‐energy dependence suggest that electron-‐phonon scattering and Auger mechanisms do not play a major role in the observed dynamics. In addition to the sub-‐ picosecond trapping, a 50 ps decay is observed in the monolayer TA dynamics. This slower decay component is associated with exciton recombination and lengthens to 150 ps in the multi-‐layer film due to the transition from direct to indirect band-‐gap. We do not observe


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signs of trion formation, which we attribute to atmospheric adsorbates that neutralize the intrinsic n-‐type behavior. Methods Sample Preparation. MoS2 films were prepared by chemical vapor deposition (CVD) in a 2 inch diameter tube furnace.12 Solid sources of MoCl5 and sulfur serve as the precursors for the MoS2 growth on c-‐plane sapphire or Si/SiO2 (275 nm).10 The growth substrate and MoCl5 precursor are placed on a quartz platform at the center of the furnace. To achieve monolayer films of MoS2, 2 mg of MoCl5 is used, with sulfur positioned at the upstream end of the furnace. Multi-‐layer (3-‐4 layer) films are achieved by increasing the MoCl5 amount to 4 mg (6 mg). The pressure is maintained at 2 Torr under a 50 sccm flow of Argon while the temperature is increased at a rate of 15° C/min. Upon reaching 850 °C, the growth temperature is maintained for 10 minutes, and the sample is then allowed to cool undisturbed under continuous Ar flow. To investigate dynamics of MoS2 on other substrates, the films were removed from the Si/SiO2 growth substrate and transferred to the desired wafer. In the first step of the transfer process, PMMA resist (950 A4) is spun onto the MoS2 and cured on a hotplate at 100°C for 10 mins. The sample is then submerged in buffered HF to etch the oxide layer, freeing the MoS2 from the growth substrate. Once released, the film is moved to DI water, where it floats on the surface. The MoS2 is then lifted from the water with the desired substrate. Adhesion is improved by spinning at 2000 rpm and baking at 100° C. The PMMA is then removed using acetone and isopropanol. Transient Absorption. A transient absorption spectrometer was used to measure the excited state dynamics in the MoS2 films. The system is based on a 1 W 150 fs Ti:Sapphire amplifier (Clark MXR, CPA). The output is split, and ~300 mW are used to pump a visible OPA (Clark MXR, NOPA) to produce tunable excitation pulses with typical incident fluences ca. 1013 photons/cm2. A small amount of power is focused into a sapphire plate to generate the white light continuum probe. The white light pulses are then sent into a scanning monochromator to record the excited state spectra. The recorded spectra is corrected for dispersion by projecting the raw data onto a Sellmeier fit to the wavelength dependence of the pump-‐probe overlap to produce a spectrum where all wavelengths have experienced the same delay. Time-Resolved THz Spectroscopy. An optical-‐pump THz-‐probe spectrometer was used to measure the photo-‐induced change in conductivity of the MoS2 films. The system is based on an 8 W 30 fs Ti:Sapphire amplifier (Coherent, Legend Elite Duo). The output is split with 3.2 W used to pump an OPA (Coherent, OperA Solo) to produce tunable excitation pulses; the remainder is used to generate and detect THz pulses. Broadband THz pulses are generated in a two-‐color air-‐plasma by focusing 1.5 W through a BBO crystal and mixing the fundamental and second harmonic in a laser-‐induced air-‐plasma filament. The THz electric field transients are detected in a 300 µm thick GaP crystal via free-‐space electro-‐ optic sampling, providing continuous bandwidth from 0.1-‐8 THz. Details of how the photoconductivity is measured via TRTS and extracted from the raw data are given elsewhere.23,24 The pump beam is focused to a ~2 mm diameter, which is larger than the 750 µm THz beam spot, and leads to typical incident fluences of ca. 1015 photons/cm2. Results



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MoS2 mono-‐ and multi-‐layer films were prepared by CVD growth.12 The number of layers was confirmed by Raman spectroscopy, atomic force microscopy (AFM), and steady-‐ state photoluminescence, as illustrated in Figure 1. AFM profiles in Figure 1b show step heights of ~1 nm and ~3 nm for the monolayer and multi-‐layer films, which is consistent with films containing 1 and 4 layers of MoS2 respectively. Further, it is also well known that the transition from indirect to direct band-‐gap occurs as MoS2 transitions from two-‐layer to monolayer.4 Strong emission is observed only for the monolayer, Figure 1c, confirming the direct band-‐gap. Importantly, the monolayer emission does not show signs of trion emission, previously reported near 1.85 eV.25 Finally, it is known that the out-‐of-‐plane A1G Raman mode blue-‐shifts and the in-‐plane E12G mode red-‐shifts as the numbers of layers increases.26 We observe splitting between the E12G and the A1G peaks of ~ 20 cm-‐1 for the monolayer and 25 cm-‐1 for the multi-‐layer MoS2 on sapphire, Figure 1d, consistent with the expected increase in peak separation.

Figure 1. a) Optical micrograph of monolayer MoS2 showing the large uniform area that results from CVD growth. A purposeful scratch in the corner shows the contrast between film and SiO2 substrate. The inset shows the monolayer MoS2 film after it is transferred onto a 1” x 1” fused silica substrate. b) AFM image of monolayer MoS2 on SiO2. The inset shows the step-‐height profile. c) Photoluminescence spectra of mono-‐ (red) and multi-‐layer (black) MoS2. d) Raman spectra of mono-‐ (red) and multi-‐layer (black) MoS2. The modes associated with the film and sapphire substrate are labeled.


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The absorption spectrum in Figure 2a shows the features typically observed for monolayer MoS2 labeled “A”, “B” and “C” at 1.89 eV (655 nm), 2.04 eV (608 nm), 2.87 eV (432 nm), respectively. Spin-‐orbit splitting of the valence band leads to the “A” and “B” features, both corresponding to excitonic transitions at the K-‐point of the Brillouin zone. The “C” feature corresponds to transitions involving bands located between the K-‐ and Γ-‐ points, and has been attributed to van Hove singularities in the density of states27 as well as to the band-‐gap of monolayer MoS2.28

Figure 2. (a) Absorbance of monolayer MoS2 on fused silica. (b) Transient absorption spectra measured as a function of delay for monolayer MoS2 photo-‐excited at 532 nm. (c) Excited state dynamics probed at 490 nm (blue), 610 nm (green), 660 nm (red), and 690 nm (dark red). (d) Fit results to the TA spectra showing the temporal evolution of the A (red) and B (blue) absorption feature amplitude, line-‐width, and center wavelength. Solid lines are exponential fits. TA measurements yield the time-‐evolution of the excited state spectrum, providing detailed information of the excited state dynamics. The excited state spectrum of monolayer MoS2 show that the change in transmission, ∆T/T0, alternates between positive and negative, Figure 2b. This type of response has been previously observed for multi-‐layer MoS214, 17-‐18 and was also recently observed in monolayer WS2.29 It is clear that such a TA spectrum cannot be described by simple band-‐filling, which would produce ground state



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bleaching that mirrors the absorption spectrum. Instead, the absorption features must shift or broaden to produce the observed response. It is tempting to interpret the alternating positive and negative changes in transmission as belonging to different optical transitions, i.e. ground-‐state bleaching and excited state absorption (ESA). This explanation is inadequate to describe our observations for the following reasons: (1) while the A-‐ trion has been observed near 670 nm in absorption and luminescence studies of doped MoS2 and at low temperature,25, 30 no similar peaks have been reported for the B and C features to explain the potential ESA near 630 nm and 490 nm; (2) there are no other known transitions that correspond to the energies associated with the negative ∆T/T0 features; (3) interpreting each TA peak as an optical transition requires an explanation for the unexpected apparent higher state filling of the B state as compared to the A state. We instead observe agreement between the decay dynamics measured for each of these features, Figure 2c, which implies they have common origins. Our interpretation of the TA spectra invokes a spectral-‐shift and line-‐width broadening, which is more consistent with not only our data but with observations reported in the literature as well. We simulate the TA spectra by fitting the absorption spectrum and then calculate the changes to the A and B absorption features, by way of amplitude changes (i.e. bleaching), line-‐width broadening (ΔΓ/Γ ), and spectral shift (Δhν), that will produce the observed change in transmission. We fit the absorption spectra with two Gaussians, one each for the A and B excitonic features. We then compute the absorption of the excited state as 2 2 −( λ −( λ 0,A +Δλ A ))2 /(ΓA R A )2 + Be −( λ −( λ 0,B +Δλ B )) /(ΓB R B ) , α exc ( λ ) = Ae 1 where Ri is the ratio of the unexcited and excited MoS2 ground state absorption line-‐widths, Γi is the ground state absorption line-‐width, and ∆λ is the change in the center wavelength λ0,i. We can then calculate the measured photo-‐induced change in the transmission spectrum a€s ΔT 10 −α exc ( λ ) = −α 0 ( λ ) −1, 2 T 10 where α0(λ) is the ground state absorption spectrum. The temporal evolution of the fit parameters for monolayer MoS2 are shown in Figure 2d. After photoexcitation, we observe a sub-‐picosecond decay in amplitude and line-‐ € width broadening. The line-‐width broadening dominates the TA spectra and can be shown to match the temporal evolution of the spectral zeroth moment i.e. the area under each excitonic feature. Line-‐width broadening also accounts for the higher amplitude of the B feature in the TA spectrum. An increase in B excitonic line-‐width leads to increased absorption that overlaps with the A exciton feature, giving the appearance that the A exciton is less populated, see Supporting Information for more details. Over this same time-‐ scale, a small red-‐shift grows in. This is counterintuitive, as the TA spectrum appears to blue-‐shift. However, it is readily apparent that a blue-‐shifted absorption spectrum would produce a positive ∆T/T0 feature near 690 nm, while instead we observe a negative ∆T/T0 feature. Further, a shifting absorption spectrum will not create a shifting TA spectrum but instead creates an oscillatory TA spectrum that changes magnitude with the size of the shift in absorption wavelength, see Supporting Information for details. Instead, changes in line-‐ width and center wavelength combine to produce an apparent blue shift in the TA


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spectrum. The measured red-‐shift is too small to be accounted for by trion formation, which results in a ~40 meV red-‐shift of luminescence.25 After 5 ps, all fit parameters decay at the same rate. A similar analysis of transient absorption in multi-‐layer MoS2 presented in the literature concluded that the observed spectra were produced by coherent interactions between the A and B excitons based primarily on the identical A and B exciton dynamics.14 Although our spectral bandwidth is insufficient to quantify changes to the C feature, the negative ∆T/T0 feature near 490 nm suggests that the C feature undergoes similar spectral changes as the A and B features. The matching dynamics measured for 490 nm, 610 nm and 690 nm also imply a link between changes in the A, B and C features. Very recently, all three features were observed in the excited state spectrum after resonant excitation of the A exciton.31 These observations appear to be inconsistent with the interpretation that coupling between the A and B excitons gives rise to the observed TA spectrum in monolayer MoS2. Examining the dynamics at individual wavelengths associated with features in the TA spectrum provides information about the time-‐scale of the different photo-‐physical processes in monolayer MoS2. Measurements reveal that dynamics take place on two distinct times scales, Figure 2c. The B exciton dynamics, probed at 610 nm, clearly show the two decay components, which are present in both mono-‐ and multi-‐layer MoS2, see Supporting Information. The fast component decays on a sub-‐picosecond time scale with a time constant of 670 ± 110 fs, which agrees with the results obtained by fitting the TA spectra. Because of the B-‐feature line-‐width broadening, only this fast component can be resolved when probing the A exciton dynamics at 660 nm. No oscillations in the A or B exciton dynamics are observed, which would be expected if coherent interactions were present. Dynamics probed far from the excitonic features do not show this fast component because they are relatively unaffected by the rapid changes in line-‐width. Instead, only the slow decay component is observed when the dynamics are probed at 490 nm or 690 nm. The slow decay component depends on film thickness, with 50 ± 20 ps and 150 ± 40 ps time constants for the mono-‐ and multi-‐layer films respectively, Figure 3a.



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Figure 3. (a) Excited state dynamics of multi-‐ (black) and monolayer (red) MoS2 photo-‐ excited at 532 nm and probed at 490 nm. (b) Photoconductivity dynamics of multi-‐ (black) and monolayer (red) MoS2. (c) Comparison between the photoconductivity dynamics measured with TRTS and the excited state dynamics probed at 610 nm resulting from 480 nm excitation of monolayer MoS2. (d) The frequency-‐dependent photoconductivity of monolayer MoS2 on a TOPAS® substrate measured at 15 K. The lines are fits to the Drude-‐ Smith model. The photo-‐induced conductivity dynamics of the mono-‐ and multi-‐layer MoS2 films were measured using TRTS. The conductivity dynamics are similar for both the monolayer and multilayer films, Figure 3b. An ultrafast increase in conductivity was observed that decays with a time constant of 660 ± 150 fs. This is similar to the dynamics recently reported by Docherty et al.22 On the other hand, this is in sharp contrast to trion dominated dynamics reported by Lui et al.,20 where a photo-‐induced decrease in conductivity was observed. Though a similar fast component was also present in those dynamics, it was attributed to electron-‐phonon scattering after laser heating of the lattice. Such a mechanism cannot be operative here because the photo-‐induced conductivity increases. Instead, we observe that the photoconductivity dynamics precisely match the sub-‐ picosecond decay observed with TA, Figure 3c, suggesting a common origin. The frequency-‐dependent complex conductivity in Figure 3d shows non-‐Drude behavior characterized by a significant real component and a small negative imaginary component. This is characteristic of charge carrier localization and scattering due to nanoscale disorder.32 The non-‐zero real conductivity is a clear departure from the THz signature of excitons.23 The observed conductivity is also quite different from the Drude-‐ like response observed in bulk MoS2 using TRTS.33 One explanation for the observed localization is carrier backscattering at defects or grain boundaries in our polycrystalline


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film. Anderson localization, where random disorder causes destructive interference among carrier scattering events, leads a non-‐Drude conductivity that can instead be described by the phenomenological Drude-‐Smith model.34 A best fit line using the Drude-‐Smith model yields a carrier scattering time of 23 fs, a plasma frequency of 251 THz, and a persistence of velocity of -‐0.65. A persistence of velocity between 0 and -‐1 indicates partial localization. The DC mobility is estimated to be 23 cm2/Vs, assuming an effective mass of 0.61me, which is in agreement with thin-‐film transistor measurements.35 Alternatively, surface plasmon resonances or intraband transitions, in conjunction with a Drude metal response, could describe the observed conductivity. However, surface plasmon resonances are expected only at visible wavelengths for MoS2 nanoparticles, making such a model improbable. Also, there is no expected intraband transition in MoS2 between 2-‐4 THz: the valance band splitting could only produce transitions near 36 THz (150 meV) and we see no resonance associated with the 0.75 THz (3meV)36 conduction band splitting. The photo-‐induced conductivity appears to be temperature independent, with matching dynamics at 300K and 15K. We do not observe a resonance near the trion binding energy of 20 meV (5 THz), even at low temperature, which is expected if trions are present.

Figure 4. (a) Incident fluence-‐dependence of the photoconductivity dynamics for 480 nm excitation. (b) Incident fluence-‐dependence of the transient absorption spectrum measured at a delay of 5ps for 387 nm excitation. Fluences are given in photons/cm2. (c) The



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photoconductivity dynamics measured for 400 nm, 480 nm, and 800 nm excitation. (d) The excited state dynamics measured at a probe wavelength of 490 nm for excitation wavelengths of 387 nm, 480 nm, 532 nm, 615 nm, 660 nm, and 775 nm. The dependence of excited state and photoconductivity dynamics on excitation fluence and photon energy often aids in understanding underlying photo-‐physics. The photo-‐induced conductivity decay time constant was insensitive to the incident pump fluence over the range 2 – 8 x1015 photons/cm2, Figure 4a. It is important to note that at 480 nm monolayer MoS2 has ~91% transmission so that the absorbed fluence remains below the atomic sheet density. The TA spectrum, however, appears to red-‐shift with incident fluence over the range 0.1 – 1.0 x1014 photons/cm2, similar to what has been observed in multi-‐layer films, Figure 4b.18 Increased line-‐width broadening and red-‐shift of the absorption spectrum combine to produce the apparent red-‐shift of the TA spectra, see Supporting Information for details. This complicates any examination of the fluence dependence of individual spectral features probed at a specific wavelength. It is worth noting that the absorbed fluence in all cases is within an order of magnitude of the reported Mott density (1 x 1013 cm-‐2) in monolayer MoS2.5 The photo-‐induced conductivity dynamics were also insensitive to changes in photon energy, showing a similar response for sub-‐ optical gap excitation at 800 nm (1.55 eV), Figure 4c. This requires a mechanism for charge carrier generation at energies below the ~1.9 eV optical gap. Similarly, the excited state dynamics are found to be insensitive to excitation photon energy, see Supporting Information for details. Probing at 490 nm, we see that the slow component of the excited state dynamics is independent of excitation energy, even for 775 nm (1.6 eV) excitation, Figure 4d. This points to the existence of mid-‐gap states in monolayer MoS2. Discussion The photoconductivity dynamics show that charge carriers, and not excitons, are responsible for the sub-‐picosecond TA dynamics. Charge carrier trapping causes rapid changes in amplitude and line-‐width broadening of both the A and B absorption features. The excitation wavelength and fluence independence does not support the involvement of carrier-‐phonon scattering or Auger mechanisms. The red-‐shift of the absorption spectrum that grows in is due to band-‐gap renormalization associated with the Coulombic interactions within the 2D film. Here, changes to both the electronic bandgap and exciton binding energy only partially compensate, giving rise to the observed red-‐shift.31 One explanation for charge generation in a two-‐dimensional monolayer film with strong Coulombic interactions is that the excitation fluences necessitated by these experiments approach the Mott density, above which band-‐like transport is expected.5, 37 Similar dynamics observed in multi-‐layer films, where the Mott density is not known, may suggest that an intrinsic charge separation pathway could also exist. We propose that sub band-‐gap charge carrier generation occurs by photo-‐excitation of mid-‐gap trap states, likely located at surface defects and grain boundaries. Grain boundaries38 and vacancies39 have been theorized to produce mid-‐gap states in monolayer MoS2. Our CVD growth from a MoCl5 precursor results in lower emission intensities and smaller grains sizes as compared to exfoliated flakes or growth from MoO3 precursor, see Supporting Information for details, which also suggests a correlation between trap density and grain boundary density. The observed trapping dynamics require defect densities to be


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near the injected carrier density of ca. 1013 cm-‐2, which is in good agreement with recent theoretical and experimental estimates of the defect density associated with grain boundaries40 and vacancies.37, 41 This is consistent with the observed wavelength independent charge carrier generation and excited state dynamics. Because the observed dynamics are unchanged for fused silica and TOPAS® polymer substrates, we do not suspect that the trapping occurs at the substrate-‐film interface. We observe no appreciable change in luminescence intensity upon successive transfers, suggesting that the defect density does not increase during the transfer process and that the defects are intrinsic to the film. Therefore we propose that charge carriers are partially localized by backscattering effects either due to the potential barrier at the grain boundary or the potential well of a lattice defect, leading to rapid trapping within 600 fs. Interestingly, these trapping mechanisms dominate the optoelectronic properties of both mono-‐ and multi-‐layer MoS2. We assign the slow TA decay to tightly bound excitons. This is consistent with their absence from the THz conductivity. Excitons exhibit zero real conductivity at THz frequencies because the THz electric field cannot drive the motion of neutral particles and so there will be no energy dissipation. Instead, excitons interact with THz via their polarizability, leading to a capacitive response through the imaginary conductivity.23 Therefore, tightly bound excitons that lack sufficient polarizability will not be detected at THz frequencies. These excitons can partially occupy and be localized by defects states, which has led to previous reports of sub band-‐gap emission in monolayer MoS2,42 or may remain after traps are filled by mobile charge carriers. The strong Coulombic interactions that give rise to bound excitons also give rise to band-‐gap renormalization, so that the observed red-‐shift is proportional to the density of photo-‐excited excitons. The 50 ps decay time for the monolayer is consistent with previous reports of the luminescence lifetime in MoS2.19 The increase in exciton lifetime from monolayer to multilayer films may appear consistent with the transition from direct to indirect band-‐gap, because electron-‐phonon interaction required in indirect band-‐gap semiconductors leads to longer radiative lifetimes. However, recent measurements of the thickness-‐dependence of the exciton lifetime in MoS2 show that fast surface recombination dominates slow bulk recombination in few-‐layer MoS2.43 So we instead assign the shorter exciton lifetime in monolayer MoS2 to the surface recombination limited exciton lifetime. We see no signs of trions in the MoS2 films. We do not observe a photo-‐induced decrease in the conductivity associated with trion formation nor a resonance near 20 meV (i.e. 5 THz) associated with the trion binding energy. The reason for the absence of trions may be related to the grain size of our MoS2 films. CVD-‐growth using MoCl5 as a precursor typically results in polycrystalline MoS2 films with small grains, ca. 10 nm, as compared to the large grains, ca. 1 µm, found in mechanically exfoliated flakes9-‐10 or by using MoO3 as a precursor, see Supporting Information for details. The intrinsic n-‐type doping of MoS2 is neutralized by atmospheric adsorbates,42, 44 e.g. oxygen, which preferentially attach at grain boundaries,45 such that films with smaller grains are less likely to show n-‐type behavior. Without the intrinsic n-‐type doping, negative trions do not form. Recently, it was shown that trions or excitons can be selectively prepared in WS2 by cleaning the surface of adsorbates,46 but adsorbates may bind more strongly to MoS2. Comparatively, the recent study of trions in CVD grown MoS2 by Lui et al. involved films with large grains formed through seeded CVD-‐growth,11, 20 where the intrinsic n-‐type doping was maintained. This may explain the recent conflicting reports of conductivity dynamics observed via TRTS in



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CVD-‐grown monolayer MoS2. Direct measurements comparing the photoconductivity of monolayer MoS2 films with different grain sizes would aid in confirming this supposition. Conclusions In summary, we have observed an ultrafast photoconductivity decay that matches a corresponding absorption line-‐width broadening, red-‐shift, and decay of ground state bleaching. We assign these dynamics to charge carrier trapping due to mid-‐gap states associated with grain boundaries or surface defects. This points to the importance of growth procedures that yield high quality films with large grain size for optoelectronic devices based on monolayer TMDs. On the other hand, such a fast conductivity response makes TMDs a good candidate for high-‐speed optoelectronics including photodetectors, modulators, and THz photoconductive dipole antennae. We also observe a longer-‐lived excited state that has no associated conductivity to produce THz attenuation, which we assign to the decay of tightly bound excitons. This exciton lifetime depends on film thickness due to surface recombination, which dominates the exciton lifetime. The absence of trions is attributed to atmospheric adsorbates that neutralize the intrinsic n-‐type character. Similar observations may also be expected for the neutral excited states of other CVD-‐grown TMD monolayer films. Supporting Information Available: details concerning the modeling of transient absorption spectra, fluence dependence of the transient absorption spectra, excitation photon energy dependence of transient absorption spectra, substrate and film thickness dependence, temperature dependence, and a comparison of different sample preparation techniques. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement: This work was supported by core programs at the U.S. Naval Research Laboratory (NRL), the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research under contract number AOARD 14IOA018-‐134141. KMM acknowledges the Nation Research Council research associates program. References:

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