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Letter

Auger Recombination in CVD-grown Monolayer WS2 Paul D. Cunningham, Kathleen M. McCreary, and Berend T. Jonker J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02413 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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The Journal of Physical Chemistry Letters

Auger Recombination in CVD-grown Monolayer WS2 Paul D. Cunningham*, Kathleen M. McCreary, Berend T. Jonker U.S. Naval Research Laboratory, Washington, DC 20375 *[email protected]

ACS Paragon Plus Environment

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ABSTRACT

Reduced dimensionality and strong Coulombic interactions in monolayer semiconductors lead to enhanced many-body interactions.

Here, we report Auger recombination, i.e. exciton-exciton

annihilation, in large-area CVD-grown monolayer WS2. Using ultrafast spectroscopy we experimentally determine the Auger rate to be 0.089 ± 0.001 cm2/s at room temperature, which is an order of magnitude larger than the bulk value. This nonradiative recombination pathway dominates, regardless of excitation energy, for exciton densities greater than 8.0 ± 0.6 x 1010 cm2

and below the Mott density. Higher energy excitation above the A exciton resonance may

initially produce a hot electron-hole gas that precedes exciton formation. Therefore, we use resonant excitation of the A exciton to ensure accuracy and avoid artifacts associated with other photogenerated species.

TOC GRAPHICS


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Monolayer transition metal dichalcogenides (TMDs) have attracted much interest as a new class of atomically thin two-dimensional materials beyond graphene. Unlike graphene, these materials exhibit a significant bandgap. Since they become direct-gap semiconductors in the monolayer limit,1, 2 TMDs are of particular interest for optoelectronic applications. Reduced screening of the electric field enhances Coulombic interactions and leads to the formation of excitons and trions with binding energies3-6 much larger than typical two-dimensional semiconductors such as layered quantum well structures, making these materials efficient light emitters and single photon sources.7-10 Under certain conditions, lasing has been achieved in monolayer TMDs.11, 12 Reduced dimensionality can also enhance many-body interactions, including the process of Auger recombination (AR). In AR, one exciton nonradiatively recombines and its energy is transferred to another exciton. This process is operative only at very high charge densities in bulk materials, but becomes an important loss mechanism in low dimensional semiconductors such as quantum dots and rods.13, 14 Efficient AR, a.k.a. exciton-exciton annihilation, has been found in two-dimensional MoS2,15 MoSe2,16 WSe2.17 However, conflicting reports exist for WS2, where AR has been reported to be absent,18 modestly efficient,19 and highly efficient.20 As this nonradiative recombination pathway can limit the efficiency of light emitting diodes and inhibit a population inversion needed to achieve lasing, clarification of the Auger rate is needed. In this letter, we report Auger recombination (AR) in large area CVD-grown monolayer WS2. We utilize ultrafast transient absorption spectroscopy to monitor the exciton recombination dynamics as a function of both excitation density and energy. We find that AR is present regardless of the excitation energy. We uniquely utilize resonant excitation of the A exciton, to minimize artifacts associated with other photogenerated species that are excited at higher energies. We experimentally determine the Auger rate to be 0.089 ± 0.001 cm2/s. Based on this


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rate, AR will dominate the exciton dynamics for exciton densities larger than 8.0 ± 0.6 x 1010 cm2

and below the Mott density.


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Figure 1. Fluence dependence of the transient absorption of monolayer WS2. (a) The absorption spectrum of monolayer WS2 indicating the A-, B-, and C- excitonic features. (b) The transient absorption spectrum near the A exciton feature measured 5 ps after photoexcitation is shown as a function of fluence for resonant excitation of the A exciton. The arrow indicates the large ground-state bleach region. (c) The peak amplitude of the photoinduced A exciton bleach (at ~ 618 nm) is shown as a function of fluence. The transient absorption (TA) spectrum of monolayer WS2 in Figure 1a shows a clear bleach ∆/ > 0 feature corresponding to A excitonic absorption, where the absorption is reduced due to state filling. The two trion features that have previously been reported21 are absent here. Because vacuum thermal22 and laser annealing23 helps induce a trion dominated photoresponse, the lack of trions here may arise from surface adsorbates that neutralize negatively charged defect sites. Changes in linewidth and a spectral shift, which stem from a combination of Pauli blocking,24 changes in exciton binding energy, and bandgap renormalization,25 cause the characteristic dispersive TA spectrum26 that displays both oscillation from bleach to photoinduced absorption and a small spectral shift with respect to the ground state absorption spectrum. Similar spectral modifications have been previously observed in monolayer MoS2.27 However, unlike in MoS2, the shape of the excited state spectrum shows negligible evolution with time, see Supporting Information, and the dynamics are instead dominated by changes in amplitude. We therefore treat the peak amplitude of the A exciton bleach as directly proportional to the exciton density. In the absence of charge carriers each absorbed photon creates an exciton so that the initial exciton density is simply the excitation fluence. The amplitude of the A exciton bleach in Figure 1b shows saturation behavior with increasing excitation fluence. Fitting the fluence dependence with the saturable absorber model


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∆

∝ ,

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(1)

where n is the exciton density, and is the saturation density, we arrive at a saturation density of 3.4 ± 0.4 x 1012 cm-2, which corresponds to an exciton-exciton distance of 54 ± 3 Å. For fluences below the saturation density the measured bleach dynamics are dominated by changes in the exciton population. Above this saturation density, other photoexcited species may be generated. For example, this saturation behavior may be caused by approaching the Mott density, which has been reported to be between 6 - 100 x 1012 cm-2.28, 29 There, significant charge carrier densities screen the Coulombic interaction between electrons and holes leading to changes in the observed TA spectra and excited state dynamics. We also observe a small blue shift of the TA spectrum with increasing fluence, Figure 1b. Comparing the lowest and highest fluences applied, we see that the peak of the A exciton bleach shifts by approximately 2 nm. This differs from reports of an increasing redshift of the TA spectrum observed at high fluences (> 1 x 1014 cm-2) in MoS2, which was assigned to a chargecarrier induced Stark effect.30 Here in WS2, the blue shift may be due to the Burstein-Moss effect, where Pauli blocking of the band edge states pushes excitonic transitions to higher energies. Alternatively, it has been suggested that changes in the exciton binding energy due to screening and band gap renormalization compete to produce a small net blue shift.28 Examining the A exciton bleach dynamics, Figure 2, we observe that a fast component grows and dominates the exciton lifetime as the excitation fluence is increased. This behavior is independent of excitation energy and occurs for resonant excitation of the A exciton (615 nm) and B exciton (515 nm) as well as for excitation at 3.2 eV (387.5 nm), which is above the C


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excitonic feature. A small peak about 5 ps after the main peak, Figure 2b, is due to reflection of the pump pulse at the substrate-air interface. For excitation above the A exciton energy, Figure 2b-c, we see an additional sub-picosecond decay. This sub-picosecond feature is independent of fluence but becomes larger for increased excitation energy. This feature may be due to creation of a hot electron-hole gas that quickly cools and forms excitons. This is consistent with reports that the B exciton absorption is coincident with the ionization energy of the A exciton.3 Free charge, which has twice the reverse saturation density as excitons,31 is more efficient at bleaching the ground state absorption and could therefore produce this short-lived peak. Carrier thermalization, which has been reported to take place in 10s of femtoseconds in two-dimensional TMDs,32 is far too fast to produce the observed decay. The lack of a fluence dependence and the observed energy dependence also suggests that ultrafast trapping27, 33 is not responsible for the observed decay. A similar feature observed in MoSe2 displays a threshold wavelength and has been assigned to exciton formation.31 Therefore, we tentatively assign this sub-picosecond decay to exciton formation from an initially hot electron-hole gas. The consequence of this interpretation is that the exciton biding energy may have an upper limit of approximately 400 meV, which is much smaller than the values typically reported4 and in better agreement with recent estimates based on Rydberg states3 and diamagnetic shifts.34 More detailed wavelength dependent measurements are needed to confirm the origin of this sub-picosecond decay in WS2.


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Figure 2. Fluence dependent A exciton dynamics in monolayer WS2 for different excitation wavelengths. These dynamics were measured for (a) resonant excitation of the A exciton at 614 nm, (b) resonant excitation of the B exciton at 515 nm, and (c) for excitation above the C exciton at 387.5 nm. The arrows in (a) highlight the fluence-dependent fast recombination pathway, in


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(b) indicate a feature caused by the reflected excitation pulse, and in (c) indicate the subpicosecond decay present for excitation energies above the A exciton resonance.

Figure 3. Fluence dependence of the Auger-limited exciton lifetime in monolayer WS2. This lifetime was determined by single exponential fits to the data in Figure 2a. The solid line is a guide.

Fitting the A exciton bleach dynamics that resulted from resonant excitation with a single exponential, Figure 3, we see that the exciton lifetime decreases with pump fluence. This suggests that a higher order decay mechanism is operative in this fluence range. In low dimensional semiconductors, AR can become a dominant nonradiative decay channel. In the presence of AR, the exciton population follows the relation

,

(2)

where n is the exciton density, is the exciton lifetime, and is the Auger rate.


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Figure 4. Auger recombination of excitons in monolayer WS2. Dots are the A exciton dynamics measured for resonant excitation of the A exciton at 614 nm for different excitation fluences: 2 x 1012 cm-2 (blue), 7.9 x 1011 cm-2 (green), 3.9 x 1011 cm-2 (red), and 2.0 x 1011 cm-2 (black). The solid lines are fits to the solution of Equation 2 as described in the text.

We fit the exciton dynamics to the solution to Eqn 1 with the Auger rate and exciton lifetime as global variables. We find an exciton lifetime of 140 ± 10 ps and an Auger rate of 0.089 ± 0.001 cm2/s. Applying this analysis to the A exciton dynamics resulting from 387.5 nm and 515 nm excitation yields similar Auger rates of 0.08 and 0.07 cm2/s respectively, though in these cases the presence of the additional subpicosecond decay complicates the dynamics and increases the fit uncertainty. The measured Auger rate is an order of magnitude higher than the Auger rate in bulk20 WS2 and twice the Auger rate found in monolayer MoS2.3 From this rate, we estimate that AR will dominate for injected exciton densities > 1⁄ ⁄ , which corresponds to > 8.0 ± 0.6 x 1010 cm-2. In principal, this process should inhibit the formation of a population inversion by providing a rapid nonradiative recombination route whose rate scales with excited state density. However, it has been shown that the Mott insulator metal transition that occurs at high


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fluences shifts the Fermi level to below the A exciton energy and leads to a population inversion.29 The Auger rate determined here differs dramatically from two prior reports. Yuan et al. found exceptionally efficient AR in WS2 exfoliated flakes.20 The source of this discrepancy may be differences in the exciton diffusion rate between CVD and exfoliated WS2, which can affect Auger processes by changing the rate that excitons come into contact. Also, it has recently been observed that removal of the underlying substrate increases the Auger rate by way of changing the local dielectric environment,19 which changes the exciton binding energy and thereby the interaction radius of the excitons. On the other hand, He et al. found that AR was absent from WS2 exfoliated flakes.18 Larger emission widths found in those samples may indicate high defect densities. Defects can lead to non-radiative recombination pathways that compete with Auger processes and obscured their identification. Our measurements agree well with a recent report from Yu et al,19 and clarify the discrepancy among published results. However, unlike all previous studies, we uniquely utilize resonant excitation of the A exciton. As we have shown, this avoids complications associated with the generation of other photoexcited species that may decay on dramatically different time scales and artificially influence the extracted Auger rate. In conclusions, we have presented ultrafast transient absorption measurements of the exciton dynamics in WS2. We observe Auger recombination, which limits the exciton lifetime for the range of fluences used. This process is present regardless of excitation energy, though excitation energies greater than or equal to the B exciton energy appear sufficient to ionize excitons so that we observe a short-lived decay potentially due to hot electron-hole gas that precedes exciton formation. Therefore, the most accurate measurement of the Auger rate of exciton annihilation in TMD films requires resonant excitation of the A exciton, which we implement. As other lo-


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dimensional materials may behave similarly, this method can be applied generally more generally beyond TMDs to other systems as well. The measured Auger rate of 0.089 ± 0.001 cm2/s implies that Auger recombination is the dominant decay pathway for exciton densities larger than 8.0 ± 0.6 x 1010 cm-2. These measurements clarify previous disagreement in the literature. This threshold should be kept in mind when designing light emitting diodes based on WS2 as Auger recombination will decrease the luminescence efficiency at higher exciton densities. Experimental Methods Monolayer WS2 on SiO2/Si was prepared by chemical vapor deposition in a 2 inch tube furnace. WO3 powder, and sulfur precursors are heated to 850 C under a 100 sccm argon and 10 sccm hydrogen flow. Perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt is used as seed molecules to promote lateral growth. This procedure was recently shown to produce large (>20 µm) grain uniform coverage monolayer WS2 that exhibits enhanced photoluminescence (PL).35 After growth, the WS2 film was transferred onto a fused silica substrate for transmission measurements using a wet transfer technique.27 The monolayer nature of the film was confirmed by Raman and PL mapping, see Supporting Information for details. The PL spectrum is dominated by the A exciton emission and shows no significant trion character. The transient absorption spectroscopy setup has been detailed elsewhere.27 Monolayer WS2 films were photoexcited using tunable pulses from an optical parametric amplifier (Clark NOPA) and probed with a white-light continuum that was analyzed using a scanning monochromator. To eliminate artifacts in the TA spectra due to scatter from the excitation beam, simultaneous modulation of the pump and probe beams was employed with lock-in detection at the sum of the


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two modulation frequencies. Films were kept under dry air flow at room temperature during all measurements. ACKNOWLEDGMENT 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. REFERENCES 1. Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271-1275. 2. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New DirectGap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. 3. Chernikov, A.; Berkelbach, T. C.; Hill, H. M.; Rigosi, A.; Li, Y.; Aslan, O. B.; Reichman, D. R.; Hybertsen, M. S.; Heinz, T. F. Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2. Phys. Rev. Lett. 2014, 113, 076802. 4. Zhu, B.; Cui, X. Exciton Binding Energy of Monolayer WS2. Sci. Reports 2015, 5, 9218. 5. Ramasubramaniam, A. Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 115409. 6. Hanbicki, A. T.; Currie, M.; Kioseoglou, G.; Friedman, A. L.; Jonker, B. T. Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2. Solid State Commun. 2015, 203, 16-20. 7. Srivastava, A.; Sidler, M.; Allain, A. V.; Lembke, D. S.; Kis, A.; Imamoglu, A. Optically active quantum dots in monolayer WSe2. Nat. Nanotechnol. 2015, 10, 491-496. 8. He, Y.-M.; Clark, G.; Schaibley, J. R.; He, Y.; Chen, M.-C.; Wei, Y.-J.; Ding, X.; Zhang, Q.; Yao, W.; Xu, X.; Lu, C.-Y.; Pan, J.-W. Single quantum emitters in monolayer semiconductors. Nat. Nanotechnol. 2015, 10, 497-502. 9. Koperski, M.; Nogajewski, K.; Arora, A.; Cherkez, V.; Mallet, P.; Veuillen, J.-Y.; Marcus, J.; Kossacki, P.; Potemski, M. Single photon emitters in exfoliated WSe2 structures. Nat. Nanotechnol. 2015, 10, 503-506. 10. Chakraborty, C.; Kinnischtzke, L.; Goodfellow, K. M.; Beams, R.; Vamivakas, A. N. Voltage-controlled quantum light from an atomically thin semiconductor. Nat. Nanotechnol. 2015, 10, 507-511. 11. Ye, Y.; Wong, Z. J.; Lu, X.; Ni, X.; Zhu, H.; Chen, X.; Wang, Y.; Zhang, X. Monolayer excitonic laser. Nat. Photonics 2015, 9, 733-736. 12. Wu, S.; Buckley, S.; Schaibley, J. R.; GFend, L.; Yan, J.; Mandrus, D. G.; Hatami, F.; Yao, W.; Vuckovic, J.; Majumdar, A.; Xu, X. Monolayer semiconductor nanocavity lasers with ultralow thresholds. Nature 2015, 520, 69-72.


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