Tuning carrier concentration in a superacid treated

Tuning carrier concentration in a superacid treated MoS2 monolayer Maciej R. Molas1,2,* , Katarzyna Gołasa1 , Łukasz Bala1,2 , Karol Nogajewski1,2 , Miroslav 1,* ´ Bartos1 , Marek Potemski1,2 , and Adam Babinski 1 Institute

of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warszawa, Poland 2 Laboratoire National des Champs Magnetiques ´ Intenses, CNRS-UGA-UPS-INSA-EMFL, 25, avenue des Martyrs, 38042 Grenoble, France * [email protected], [email protected]

ABSTRACT The effect of the superacid treatment on optical properties of MoS2 monolayers is investigated. Photoluminescence, reflectance contrast and Raman scattering spectroscopy were employed in a broad range of temperature. It has been found that the treatment results in systematic quenching of the trion emission/absorption and the redshift of the neutral exciton emission/absorption associated with both A and B excitonic resonances in monolayer MoS2 . The trion complex related to the B exciton in monolayer MoS2 has also been identified. The defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to the effective passivation of defects on the sample surface. The passivation leads to the vanishing carrier density, which affects the out-of-plane electric field in the structure. The observed tuning of carrier density influences also strongly the Raman scattering in the MoS2 monolayer. The enhancement of the Raman 0 scattering by the resonance with neutral exciton in the vicinity of the A resonance affects both out-of-plane A1 and in-plane 0 E modes. On the contrary, while the excitation is in resonance with a corresponding trion, Raman scattering can be hardly distinguished. These results confirm a role of the excitonic charge state on the resonant effect of the excitation on the Raman scattering in transition metal dichalcogenides.

Introduction Molybdenum disulfide (MoS2 ) is the most known member of semiconducting transition metal dichalcogenides (S-TMDs), which have recently attracted considerable attention due to their unique electronic structures and corresponding optical properties1–4 . As other members of S-TMDs family, MoS2 transforms from indirect- to direct-band gap semiconductor, when thinned down from bulk to a monolayer (ML)5, 6 . In consequence, S-TMDs monolayers are considered very promising building blocks of novel optoelectronic devices7–11 . The potential of S-TMDs is related to the their specific structure, which comprises strongly bound metal and chalcogen atoms within a monolayer and weak van der Waals interactions keeping the MLs together to form bulk. One of the features of the structure is the absence of dangling bonds at the terminal layer of the S-TMD. In order to profit from the unique properties, it is therefore crucial to care for the quality of the S-TMDs surface. In particular, this means a need to keep the S-TMD surface free from chemical residues and defects which can negatively affect their optical properties. Recently, it has been reported that the encapsulation of the S-TMD monolayers in hexagonal boron nitride flakes leads to suppression of the inhomogeneous contribution to the linewidths of excitonic resonances, which results in the significantly narrow spectral lines12–15 . Other approach to heal the S-TMD surface, is its chemical treatment. Notably, Amani et al.16 have shown that the treatment of MoS2 ML with an organic bis(trifluoromethane) sulfonimide superacid (TFSI), referred to as the superacid, results in an increase of the related photoluminescence (PL). The reported PL intensity grows about two orders of magnitude at room temperature. Subsequent reports17–20 show that the room temperature PL intensities of passivated MoS2 MLs grow from around one to three orders of magnitude in comparison with as-deposited (exfoliated) MoS2 samples. Moreover, it has been demonstrated that the passivation of the MoS2 monolayer suppresses the dominant defect-related emission seen in its PL spectrum measured at liquid helium temperature. However, the relative emission intensity due to the neutral and charged excitons seen at low temperature is not affected by the treatment process20 . In consequence, it can be expected that the obtained results of the passivation process, e.g. an increase of PL intensity, may depend on sample’s quality and/or experimental conditions. In order to advance the understanding of the effect of the chemical treatment on S-TMDs, we performed the investigation of optical properties of ML MoS2 subjected to the superacid treatment. We have not found any substantial increase in the

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Figure 1. Effect of the passivation processes on the (a) photoluminescence and (b) reflectance contrast spectra measured on the MoS2 monolayer at room temperature. Label "0" describes an as-deposited ML, while labels 1st . . . 4th correspond to the number of passivation process which the ML was subjected. PL intensity measured at room temperature as a result of the passivation process. On the contrary, the PL emission due to the negative trion quenches because of the treatment. A systematic energy redshift of the neutral excitons associated with both the A and B fundamental excitonic resonances at the K point of the MoS2 Brillouin zone (BZ) is apparent. We associate our observations to the passivation of unintentional doping at the surface of MoS2 monolayer and the resulting decrease in the out-of-plane electric field in the structure. The modulation of the excitonic energy affects also the Raman scattering (RS) efficiency in the studied ML. Substantial enhancement of the RS is observed, while the excitation is resonant with the neutral exciton in the vicinity of the A resonance. The RS of light in resonance with the corresponding trion can be hardly recognized.

Experimental results Carrier concentration in a superacid treated monolayer

Room temperature (T =300 K) PL and RC spectra of the MoS2 monolayer before and after subsequent four passivation processes are presented Fig. 1. A comparison of the PL spectra measured on the as-deposited ML and after the first superacid treatment allows us to recognize up to three emission lines, denoted as TA , XA and TB +XB (see Fig. 1(a)). We ascribed these peaks to the neutral (XA , XB ) and charged (TA and TB ) excitons formed in the vicinity of the so-called A and B excitons of the fundamental band gap at the K± points of the BZ due to the apparent spin-orbit splittings in both conduction and valence bands6 . As can be appreciated in Fig. 1(a), the effect of acid treatment on the PL spectrum is the most prominent after the first process. The ∼100% increase in the PL intensity is observed as a consequence of the passivation. This is in contrast to previous reports16–20 , which demonstrate at least one order of magnitude PL intensity improvement at room temperature. The second significant result of the first acid treatment is a blueshift of the emission related to the A exciton by about 30 meV. The corresponding RC spectrum of the as-deposited ML displays two resonances, which we labelled TA and TB in Fig. 1(b). A similar blueshift of around 20 meV is also apparent in the RC spectra for both the A- (TA , XA ) and B-exciton (TB , XB ) resonances after the first passivation process, which is accompanied with a gain of the resonances intensities. Moreover, the noticed blueshifts are accompanied by reduction of the linewidths of the emission/absorption resonances (see Figs 1(a) and (b)). The observed changes in both the PL and RC spectra as a result of the first acid treatment can be understood in terms of a significant decrease of the high non-intentional doping of the as-deposited monolayer. While the charged excitons (TA and TB ) mainly contribute to both the PL and RC features in the as-deposited sample, the neutral excitons (XA and XB ) dominate the PL and RC spectra after the first superacid treatment.The observed blueshifts of around 20-30 meV reasonably match the energy separation between the neutral and charged excitons (XA and TA ) reported several times for MoS2 ML20–22 , which strongly support our model. Moreover, it is well known that the oscillator strength of the neutral exciton is much bigger than that of the trion23, 24 , which is reflected in the intensities of the corresponding resonances seen in the RC spectra, see Fig. 1(b). Note that an analogous effect expected for the B-exciton emission can not be easily recognized probably due to its small intensity. 2/8

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Figure 2. Effect of the passivation processes on the (a),(b) photoluminescence, (c) reflectance contrast and (d) Raman scattering (λ =632.8 nm) spectra measured on the MoS2 monolayer at T =5 K. Label "0" describes an as-deposited ML, while labels 1st . . . 4th correspond to the number of passivation process which the ML was subjected. The red dashed vertical line in panel (b) indicates the energy of the resonant Raman excitation conditions (λ =632.8 nm). The energies of the TA and XA lines, based on the analysis of spectra shown in panel (b), are denoted by the blue and orange arrows, respectively.

The effects of the subsequent passivations (from the second one) on the PL and RC spectra is less pronounced and appears as monotonic redshifts of both the XA and XB resonances of the order of about 10 meV after the four passivation processes. This effect can be explained in terms of modification of build-in vertical electric field in the structure, i.e. quantum confinement Stark effect, due to passivation of defects on the sample surface. Note that the linewidths of the A-exciton emission, see Fig. 1(a), is also reduced from ∼90 meV before acid treatment to ∼60 meV after fourth passivation processes, which confirms the quenching of the trion emission and it is similar as already reported in Ref. 20. The effect of the superacid treatment described above can be studied in details at low temperature (T =5 K), because linewidths of excitonic emission/absorption resonances reduce significantly with decreasing temperature for S-TMDs23–25 . Fig. 2 presents the corresponding PL and RC spectra measured at low temperature. As it can be appreciated in the Figure, the observed XA and TA features both in the PL and RC spectra are well resolved at T =5 K. This allows us to analyse the effect of superacid treatment more accurately. The low temperature PL spectrum of the as-deposited MoS2 ML, shown in Fig. 2(a), is dominated by a broad emission band covering hundreds of meV, which is ascribed to defect states20, 26 . The quenching of this emission due to the four subsequent passivation steps is clearly seen. Its maximum intensity is on the comparable level as the XA intensity after the fourth passivation process. More information can be obtained by a closer inspection of the low temperature PL due to the A-exciton features (see Fig. 2(b)). The apparent peaks are attributed to the charged (TA ) and neutral (XA ) excitons. Similarly, as it has been observed at room temperature (see Fig. 1(a)), the PL signal related to the trion complex quenches leaving the neutral-exciton emission as a main feature of the spectrum as a result of the superacid treatment. A systematic redshift of the XA peak with the following treatment steps can be also noticed in Fig. 2(b). As we already proposed above, these two effects can be explained in terms of a significant decrease of the high non-intentional doping of the as-deposited monolayer and of presence of quantum confinement Stark effect due to modification of build-in vertical electric field in the structure due to passivation of defects on the sample surface. Both effects are also reflected in the RC spectra in the energy range of the A and B excitons (see Fig. 2(c)). In particular, the observed non-monotonic evolution of the A-exciton minimum in the RC spectrum supports the discussed scenario. Substantial density of charge carriers in the as-deposited structure results in the trion TA resonance dominating the RC spectrum. The neutral exciton XA can be recognized in the RC spectrum as a high-energy component of the trion minimum in the as-deposited structure. The contribution from the trion can be still observed after the first treatment as a low-energy shoulder of the XA dip, but after the following treatment steps it can hardly be distinguished. The quenching of the charge density and the evolution of the electric field in the structure also explains the behaviour of the RC minimum related to B-exciton spectral range (see Fig. 2(c)), which is analogous to the seen for the A exciton. However, due to larger linewidths of the B-exciton resonances, it can not be seen so obviously. In particular, the non-monotonic evolution of the B-exciton dip after the first treatment should be because of the quenching of charge carrier density in the structure. Therefore, the corresponding RC minimum in the as-deposited ML is mostly composed of contribution 3/8

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Figure 3. (a) Effect of the passivation processes on the Raman scattering spectrum (λ = 632.8 nm) measured on the MoS2 monolayer at T =5 K. Label "0" describes an as-deposited ML, while labels 1st . . . 4th correspond to the number of passivation process which the ML was subjected. (b) The Raman scattering spectrum (λ = 632.8 nm) measured on the MoS2 monolayer after 4th passivation process at T =5 K. The phonons correspond to M point of the Brillouin zone unless stated otherwise. associated with the absorption of the charged exciton TB . The RC features in the superacid-treated structures are due to the neutral excitons (XB ), which undergo quantum confined Stark shift. It is worth to point out that the observed redshift associated with the modification of build-in vertical electric field is similar for both the XA and XB related resonances being equal to 13 meV and 10 meV, see Fig. 2(b). Note that the analogous effect to described above can be also seen in the PL experiments for the the emission lines in the vicinity of the A exciton, XA and TA , presented in Fig. 3(c). For this case, however, an emission of trion counterpart is not fully vanished from the PL spectra even after fourth passivation process probably due to the small residual doping enough to observe an emission of a charged exciton in contrast to its absorption. One of a important question is a sign of apparent charged exciton resonance. A trion, as a complex of an electron-hole pair and an extra carrier (electron or hole), can be negative (two electrons + a hole) or positive (an electron + two holes). In our case, the identification of trion’s sign can not be carried out unquestionable. Unfortunately, the same effects of the superacid treatment for resonances observed in the vicinity of both A and B excitons do not exclude formation of negatively or positively charged excitons. Resonant Raman scattering - the effect of the charge state Due to the performed four superacid treatments, the XA energy is tuned of around 30 meV, which is comparable to the energy separation between the energies of the charged and the neutral excitons. Consequently, as can be seen in Fig. 2(d), the energy of the XA emission line after fourth passivation process coincides with the TA energy for the as-deposited ML. Therefore, an unique opportunity to study Raman scattering in variable resonance conditions using a fixed excitation wavelength can be achieved. Usually employed for the resonant Raman scattering in MoS2 is the λ =632.8 nm light of He-Ne laser. It was previously shown that this excitation leads to resonant enhancement of the A1g line in bulk MoS2 27 and in multiphonon Raman scattering28 both observed at low temperature. The effect of the excitation on ML MoS2 was also studied29–32 , but the opportunity to modulate the resonant conditions by changing the density of carriers in the investigated structure was not heavily explored. This approach is employed in our investigation.

The effect of the superacid treatment on the low temperature emission excited with λ =632.8 nm light is shown in Fig. 2(d). It can be seen in the Figure that the excitation matches the neutral exciton energy in the as-deposited structure. At lower energy the emission related to trion TA can be recognized as a structure superimposed on a broad peak at ∼1.921 eV. As it was discussed earlier in the text, the trion disappears from the spectrum as a result of the superacid treatment, which is accompanied by the neutral exciton redshift. The structure due to the Raman scattering becomes more and more visible because of the applied treatment. The spectra shown in Fig. 2(d), are also presented in Fig. 3(a) in more convenient scale, as a function of the 0 Raman shift. The most intense Raman peaks in the MoS2 ML are related to two Raman-active modes: E , which results from 0 in-plane vibrations of two S atoms with respect to the Mo atom and A1 which is due to the out-of-plane vibrations of S atoms in opposite directions33 . Surprisingly, the Raman scattering signal measured in the as-deposited monolayer at T =5 K is extremely 4/8

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Figure 4. The temperature evolution of the resonant Raman scattering signal (λ = 632.8 nm) measured on the (a) as-deposited (0) and (b) fourth-passivation-subjected (4th ) MoS2 monolayers. The spectra in panel (a) are multiplied by the specified factors. The black diamonds in panel (b) indicate the XA energy found from the PL spectra measured under a 514.5 nm excitation. The 0 0 temperature evolution of the E and A1 modes intensities and their relative intensity are shown in the inset to panel (b). weak, despite the excitation energy equals almost the XA energy (see Fig. 2(c)). This leads to the conditions that the energies of the trion emission and of the first order Raman processes are overlapped, which we call a resonance with the charged exciton. When the subsequent acid treatments are implemented on the MoS2 ML, the intensity of the Raman scattering signal increases 0 significantly (for A1 line, it is of about one order of magnitude). Other striking feature of the low-temperature spectra is the presence of the emission band at ∼200 cm−1 , see Fig. 3(a). There are no center-zone Raman modes expected at that energy range in ML MoS2 . The lineshape of the emission band however corresponds to the total integrated density of phonon states in the structure34 . This suggests that single-phonon processes from outside the center of the BZ are allowed in the scattering processes. Two maxima, seen particularly in the spectrum presented in Fig. 3(a), correspond to the transverse acoustic (TA) and longitudinal acoustic (LA) phonons near the M point from the border of the BZ. Most likely origin of their presence in Raman scattering spectrum is the disorder in a structure, which localizes phonons35–37 . The attribution of the observed Raman spectrum to disorder in the structure does not explain the substantial enhancement of the Raman peaks seen at low temperature due to the subsequent implemented superacid treatments. The comparison of spectra recorded from as-deposited structure with the trion- and defect-dominated emission and from the sample after the fourth passivation clearly suggests that the enhancement is associated with resonant conditions with the neutral exciton.

Another possibility to tune the resonance conditions of the Raman scattering can be achieved by variation of sample’s temperature. The optical spectra measured on the as-deposited ML and after the four passivation processes are presented in Fig. 4. As can be noticed in Fig. 4(b), intensities of all Raman scattering peaks measured on the structure after the fourth treatment decrease gradually with increasing temperature. We ascribed this effect to the shift of neutral exciton energy away from the energy of the laser light due to the temperature dependence of the band gap23–25 . Surprisingly, the quenching affects both the 0 0 A1 and E modes. This is in contrast to the reported evolution of the Raman scattering efficiency in MoS2 ML in which the 0 0 distinct exciton-phonon coupling can be explained considering the symmetries of the A1 and E phonons and of the orbitals 0 associated with the A, B and C (high energy transition at around 2.7 eV) excitons31 . Our observation shows that the in-plane E mode in ML MoS2 undergoes the resonant enhancement at low temperature despite the symmetry effect on the exciton-phonon interaction. Additional information can be gathered from the analysis of the temperature-driven modulation of the neutral exciton XA energy. Its energy, as taken from the emission spectra excited at 514.5 nm, is denoted with diamonds in Fig. 4(b). No significant enhancement of the Raman-scattering-related emission spectra takes place in the out-going resonance conditions in which the excitonic energy coincides with the scattered light. The monotonic decrease of the emission is rather related to the detuning of the excitation laser and the neutral exciton energy. This may suggest a possible scenario responsible of the observed effects. At low temperature laser energy leads to the formation of the neutral exciton at higher k-vectors, which then 0 relax to the minimum energy state at the K± points accompanied with the emissions of discrete phonon modes (e.g. A1 ). With increasing temperature, the excitonic energy shifts away from the laser energy and the relaxation involves mainly acoustic phonons which do not contribute to the discrete emission spectrum observed at low temperature. As can be seen in Fig. 4(a), 5/8

the process is not effective in structure with high carrier density. Mainly acoustic phonons assist to the relaxation processes and the discrete structure of the optical emission spectra can be hardly seen. Our observation point out the difference between electron-phonon interaction with the neutral and charged excitons as previously reported in the WS2 monolayer38 . Furthermore, we show that carrier concentration in the studied MoS2 monolayer plays a significant role for the intensity of the resonant Raman scattering signal. It is clear that our qualitative explanation should be supported by more strict theoretical analysis, which is beyond the scope of this experimental work. We believe, however, that our work can contribute to the understanding of fundamental processes in S-TMDs, which is of prime interest for their potential application.

Conclusions In conclusion we have studied the effect of superacid treatment on the optical properties of monolayer MoS2 with the aid of photoluminescence, reflectance contrast and Raman scattering spectroscopy in a broad range of temperature. We have observed that the defect-related low-energy photoluminescence disappears from the spectrum as a result of the passivation processes. Moreover, we found that the treatment results in systematic quenching of the charged exciton emission/absorption and the redshift of the neutral exciton emission/absorption associated with both A and B excitonic resonances. The modulation of the charge state of dominant excitons in ML MoS2 affects also the Raman scattering excited resonantly. It is shown that the scattering in resonance with neutral exciton is much more effective that the scattering in resonance with the trion.

Methods Monolayers of MoS2 were prepared by mechanical exfoliation of bulk MoS2 crystals (2H phase) using polydimethylsiloxanebased technique39 . The flakes of interest were first identified by visual inspection under an optical microscope and then cross-checked by Raman scattering and PL measurements at room temperature in order to unambiguously determine their thicknesses. The prepared samples containing MoS2 MLs were treated chemically in an organic bis(trifluoromethane) sulfonimide superacid (TFSI)16 . Several subsequent steps of the treatment procedure were applied to the same sample. The PL and Raman scattering measurements were carried out using λ =514.5 nm (2.41 eV) and λ =632.8 nm (1.96 eV) radiations from continuous wave Ar-ion and He-Ne lasers, respectively. The studied samples were placed on a cold finger in a continuous flow cryostat mounted on x˘y motorized positioners. The excitation light was focused by means of a 50x long-working distance objective with a 0.5 numerical aperture producing a spot of about 1 µm diameter. The signal was collected via the same microscope objective, sent through a 0.5 m monochromator, and then detected by using a liquid nitrogen cooled charge-coupled device camera. The excitation power focused on the sample was kept at 50 µW during all measurements to avoid local heating. For RC study, the only difference in the experimental setup with respect to the one used for recording the PL and Raman scattering signals concerned the excitation source, which was replaced by a 100 W tungsten halogen lamp. The light from the lamp was coupled to a multimode fiber of a 50 µm core diameter, and then collimated and focused on the sample 0 (E) to a spot of about 4 µm diameter. We define the RC spectrum as RC(E) = R(E)−R R(E)+R0 (E) × 100%, where R(E) and R0 (E) is the reflectance of the sample and of the same structure without the MoS2 monolayer, respectively.

Acknowledgements The work has been supported by the European Research Council (MOMB project no. 320590), the EC Graphene Flagship project (no. 604391), the National Science Center (grants no. DEC-2013/11/N/ST3/04067, DEC-2015/16/T/ST3/00496, UMO-2017/24/C/ST3/00119, UMO-2017/27/B/ST3/00205), the Nanofab facility of the Institut Néel, CNRS UGA, and the ATOMOPTO project (TEAM programme of the Foundation for Polish Science co-financed by the EU within the ERDFund).

Author contributions statement M.R.M, K.G and Ł.B performed optical experiments. K.G., Ł.B., K.N. and M.B fabricated the studied samples. M.P. contributed to data analysis. M.R.M. and A.B. perfomed data analysis and wrote the paper with contribution from all authors.

Additional information Competing financial interests The authors declare no competing financial interests.

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