Systematic Study of Photoluminescence Enhancement in Monolayer

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Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment Daisuke Kiriya, Yuh Hijikata, Jenny Pirillo, Ryo Kitaura, Akihiko Murai, Atsushi Ashida, Takeshi Yoshimura, and Norifumi Fujimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01425 • Publication Date (Web): 12 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment Daisuke Kiriya1,2*, Yuh Hijikata3,4, Jenny Pirillo4, Ryo Kitaura3, Akihiko Murai1, Atsushi Ashida1, Takeshi Yoshimura1, Norifumi Fujimura1 1

Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-

ku, Sakai-shi, Osaka 599-8531, Japan 2

3

JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku,

Nagoya, 464-8602, Japan 4

Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa-ku,

Nagoya, 464-8602, Japan

Key words: Transition metal dichalcogenide, Molybdenum disulfide (MoS2), Superacid, Photoluminescence, Surface modification * [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct bandgap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a treatment with a molecular acid (bis(trifluoromethane)sulfonimide (TFSI)); however, the mechanism still remains a mystery. Here we work on PL enhancement of monolayer MoS2 by treatment of Brønsted acids (TFSI and sulfuric acid (H2SO4)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents—1,2-dichloroethane (DCE), acetonitrile, and water—were studied, as they show quite different acidity in solution. All the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and H2SO4 showed similar PL enhancement (~4-8 enhancement) at the same proton concentration, indicating the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li+ from Li2SO4, instead of H2SO4, in water. Although Li and H atoms showed similar binding energy on MoS2 from theoretical calculations, Li2SO4 treatment showed little PL enhancement; only coexisting H2SO4 reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS2. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.


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INTRODUCTION Transition metal dichalcogenides (TMDCs) have attracted attention as a next-generation material due to their atomically thin structural characteristics.1-5 The structure consists of transition metals (Mo, W etc.) and chalcogen atoms (S, Se, etc) that form a three atomic layer with a thickness of about 0.7 nm.6-7 One of the representing materials is molybdenum disulfide (MoS2), which is an atomically thin semiconducting material.1, 8 The realization of a monolayer MoS2 transistor stimulated activity in this field toward the creation of atomically thin optoelectronic devices.2 Since semiconducting device performance is highly dependent on crystal quality, defect and surface engineering are important for atomically thin materials concerning extraction of intentional performance and generation of high-performance devices.5, 7, 9-13

An indicator for identifying the quality of TMDCs is photoluminescence (PL) characteristics.12, 14-16 Monolayer MoS2 has a direct bandgap and the PL characteristics are highly dependent on the physicochemical situation of the crystal, such as defect11,

14, 17

and carrier

concentration18-20. The higher the optical quality, the longer the photoexcited carrier lifetime and the stronger the PL intensity, which is important for optoelectronic devices.21 Various studies have investigated modifying the optical quality of MoS2 via gas adsorption22-23, a thermal annealing process23-24, electrostatic gating20, and lamination with electron-withdrawing materials19, 25 and molecules18. Recently, simple molecular solution processing on monolayer MoS2 showed a near-unity PL quantum yield, close to 100% under extremely low incident light conditions.26-27 The molecule used was a superacid molecule, a strong protonation agent, bis(trifluoromethane)sulfonimide (TFSI), in a mixed solution of 1,2-dichloroethane (DCE) and dichlorobenzene (DCB). The process involves immersing in the solution for 10 minutes, then


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blowing dry N2 and annealing on a hot plate.26 Importantly, the PL improvement was maintained in the air for one week. After the initial demonstration of exfoliated monolayer MoS2, it was revealed that tungsten disulfide (WS2)28 and a monolayer MoS2 grown via chemical vapor deposition (CVD)21 also showed strong PL enhancement via the TFSI treatment. Although this molecular acid treatment is quite useful for improving the optical quality of MoS2 and WS2, the number of previous examples is limited to comprehend the mechanism. In this work, we study PL enhancement of a monolayer MoS2 by treating a strong acid solution in various solvents to evaluate the protonated environment. Since strong acids dissociate with anions and protonated solvents in solutions, the types of solvents are important for controlling the acidity (Fig. 1a). The strong acid of TFSI reproduced a PL enhancement (20 times) for a monolayer MoS2 as in the previous report26. This PL enhancement in DCE is more potent than in the other lower acidity (more than 1010-fold higher pKa29) solutions in acetonitrile and water; PL enhancement in these solutions was about 6 times. By applying water, it is possible to compare the molecular TFSI acid with another inorganic acid, H2SO4, under the conditions of a leveling effect where the acid is completely deprotonated in the solution. Both monolayer MoS2 treated with TFSI and H2SO4 in water showed similar PL enhancement and this means that the proton rather than counter anions is important for the enhancement. To further clarify this point, a solution of Li+ (from Li2SO4 in Milli-Q water), instead of proton, was applied for a monolayer MoS2. Although PL enhancement was small (several times) in treatment with only Li2SO4, the addition of H2SO4 acid started to increase PL intensity, further suggesting the importance of the protonation environment. This work will lead to broadening the idea of improving the optical quality of TMDCs to further address optoelectronic applications.


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Experimental Section Exfoliated monolayer MoS2 were prepared using a scotch tape method: MoS2 bulk mineral (SPI Supplies) was exfoliated with an adhesive tape and transferred onto a 260 nm thermally grown SiO2-covered Si wafer (Mitsubishi Material Trading Co.). The exfoliated samples were immersed in 7.1×10-4 M or 7.1×10-3 M TFSI (Sigma-Aldrich), 3.55×10-4 M H2SO4 (Kanto Chemical Co., Inc.), and 3.55×10-4 M Li2SO4 (Sigma-Aldrich) solutions on a hotplate at 85 ℃ for 10 minutes followed by N2 blowing to remove the solvent, then annealing on a hotplate at 85 ℃. The water in the all experiments was Milli-Q water. PL and Raman spectra were obtained with a LabRAM HR800 equipped with an EMCCD camera (HORIBA Scientific); the diameter of the incident beam was φ~2 µm at 532 nm laser. The incident laser power was ~2.7 W cm-2 for PL measurements and ~27 W cm-2 for Raman measurements. The fitting curves were carried out on Fityk software (0.9.8. version) with Voigt functions.

RESULTS AND DISCUSSION The acid treatment procedure for monolayer MoS2 is illustrated in Figure 1b. An exfoliated monolayer MoS2 on a 260 nm thermally grown SiO2-covered Si wafer was first immersed in an acid solution, followed by N2 blowing the solvent. After further removal of the solvent, the sample was annealed at 85 ℃ on a hot plate. The samples were evaluated using PL and Raman spectroscopy with a microspectroscopy system under an ambient atmosphere. TFSI molecules were solvated in three different solvents—DCE, acetonitrile, and water (in all cases Milli-Q water)—to change the protonated environment, as seen in Table 1. The acid is dissociated in the solvent as shown in Figure 1a, and the acidity is obtained as the equation (1),


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pKa = ̶ log([HS+][A-]/[HA])

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(1)

where Ka is the equilibrium constant and [HS+], [A-], and [HA] indicate the proton (protonated solvent), counter anion, and non-dissociated acid concentration in the solution, respectively; low pKa corresponds to high acidity. The protons should be solvated as HS+ in the solution, and therefore the type of solvent is critical to determine the acidity. Previously it was reported that the acidity of TFSI in DCE was more than 1010 times stronger than in acetonitrile.29 The acidity of TFSI in water would be similar to or less than that of it in acetonitrile by considering the donor number introduced by Gutmann; water has larger donor number than acetonitrile.30-31 The PL spectra obtained from monolayer MoS2 treated with each TFSI solution (all 7.1×10-4 M, 0.2 mg/ml) are shown in Figure 2. All samples showed PL enhancement from the original (untreated, just exfoliated sample), but the degree of the enhancement was quite different in each case. For the treatment with DCE, the PL enhancement was dramatic compared to the original, 20 times stronger. This result is similar to the previously reported cases.21, 26 In contrast, TFSI treatment in acetonitrile and water cases showed about a 6-time enhancement. This difference is due to the solvent effect, and the acidity in the solution should be one of reasons for the enhancement. A highly protonated environment would be preferred for the PL enhancement in the exfoliated MoS2. In the case of previous study with CVD-grown MoS2, a sample treated with TFSI in acetonitrile showed a higher PL intensity than it did in the DCE/DCB treatment.21 The types of original crystal (how it grows and situation of defects, etc.) also would be important for the solvent selection rules, as both solutions showed quantum yields above 10% but not close to the ~100% in the CVD-grown MoS2 case. Adsorption of solvent molecules on MoS2 surface is also not negligible because the dramatic enhancement of the PL intensity was reported by vacuuming and/or annealing monolayer MoS2 to remove surface molecules, especially water showed critical


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effect.22-23 In the case of water as the solvent (in Fig. 2d), TFSI treatment showed PL enhancement, but some monolayer MoS2 treated with water required an extra annealing process over 10 minutes, which may be due to the removal of water molecules from the surface (Fig. S1). This result is the same as in the case of H2SO4 (vide infra). From the above results, the environment of the TFSI acid is important for improving the PL intensity of the monolayer MoS2, which would be derived from the acidity and interaction of solvent molecules on MoS2. Since water is the most common solvent of acids, TFSI and H2SO4 can be compared to evaluate the further importance of the proton environment and to separate the proton and anion effects for the PL enhancement. Figure 3a shows the effect of H2SO4 treatment at the same concentration as TFSI (Fig. 2d, 7.1×10-4 N, 3.55×10-4 M in H2SO4). The PL intensity monotonically increases along the longer annealing time, from 10 minutes to 40 minutes at 85 ℃. This behavior would arise from the nonvolatility of H2SO4 which kept on the surface, while water is removed from the surface as the annealing time increases. For comparison, treatment only with water was also carried out, and it showed a smaller enhancement (Fig. 3b and Fig. 3c). These results demonstrate that acid treatment is important to increase PL intensity. The resulting degree of PL enhancement with TFSI (Fig. 2c) and H2SO4 (Fig. 3a) are similar, indicating that the anion dependence is not as significant compared to the importance of the protonated environment. Figures 3d and S2 show the Raman spectra for the original and H2SO4-treated monolayer MoS2. The samples treated with H2SO4 show an upshift of A’ Raman mode at about 1 cm-1. In previous reports, AuCl3 oxidative treatment for MoS2 showed a similar Raman mode shift.32 Therefore, the H2SO4 acid treatment would include an oxidation process. Figure 3e shows PL spectra of the just exfoliated (original) and H2SO4-treated after 40 minutes annealing (the spectra


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of H2SO4-treated after 10 to 30 minutes annealing are shown in Fig. S3). The spectra can be deconvoluted into three peaks: charged-exciton A (trion; XA- around 1.83 eV), exciton A (XA around 1.88 eV), and exciton B (labeled as B around 2.0 eV). Emissions from excitons A and B correspond to direct transition from conduction band minimum to the spin-orbit split valence band maxima at K-points.8, 33 As shown in Fig. 3f and S4, the integrated intensity of XA and XAmonotonically increases with annealing time. In previous studies, counter (p-type) doping in monolayer MoS2 using acceptor molecule or gas treatment showed increase in XA signal while maintaining the XA- signal unchanged, which is not the case of the present study.18, 33-34 For further understanding of this difference, we have conducted rate-equation analyses with a threelevel model including a trion, an exciton, and the ground state (Fig. S5). Assuming that PL intensity from excitons (Iex) and trions (Itr) are respectively proportional to exciton and trion density, the following expression for Iex and Itr can be obtained under the steady-state approximation.18 ‫ܫ‬௘௫ ∝ ‫ܩ‬ൗሺߛ + Γ ሻ ௧௥ ௫ ‫ܫ‬௧௥ ∝

‫ߛܩ‬௧௥ ୻ ൘ Γ௧௥ ቀ ೣ + 1ቁ

(2)

(3)

ఊ೟ೝ

where G, γtr, Γtr, and Γx are the optical generation rate of excitons, the formation rate of trions from excitons, the trion decay rate, the exciton decay rate, respectively. As MoS2 samples are naturally doped with electrons, p-type doping leads to decrease of number of electrons in MoS2. When number of electron decreases in MoS2, PL intensity of XA should be strong because exciton decay processes, including conversion from excitons to trions and non-radiative decay via Auger process, are suppressed; the suppression of exciton decay process leads to smaller γtr


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in the equation (2). On the other hand, PL intensity of XA- would not be sensitive to the decrease in number of electrons because the suppression of trion formation process and non-radiative decay via Auger process cancel each other (in this case, both Γtr and γtr decrease in equation (3)). This means that the observed increase in PL intensity of both XA and XA- cannot be understood only by decrease of electron density in MoS2, suggesting that the present acid treatment involves the additional suppression non-radiative decay such as passivation of defects. The ratio of the integrated intensities of XA and XA- also follows the above discussion, which shows almost constant up to 30 minutes and slightly increases at 40 minutes process (Fig. 3g). The previous reports of the carrier doping in monolayer MoS2 indicated that increasing the doping level increases the ratio of XA and XA- signals,18,

33-34

therefore, the H2SO4 treatment process is

different from the previous doping cases. From the results, the acid treatment could involve a partial charge transfer (oxidation) process and also include a surface passivation to reduce the non-radiative recombination process. Protons in acid solutions form cation species that interact with the surface of MoS2 (Fig. 4a). To take account of the surface cation effect from the charged species, a non-protonated ion, Li2SO4, was investigated. For the Li2SO4 aqueous solution, the PL intensity of the monolayer MoS2 shows an enhancement of several times, which is the same tendency as in the water treatment (Fig. 3b and Fig. 4a). On the other hand, the further addition of H2SO4 to the Li2SO4 solution starts increasing the PL intensity, and its behavior is similar to the H2SO4-treated (Fig. 4c). The results of these experiments clearly show the importance of the protonated environment, not just the cationic effect, to improve the PL intensity. From the density functional theory calculations, Li and H atoms on S defects (Mo-Li and Mo-H bonds) show similar binding energies of 2.9 and 2.6 eV, respectively (Fig. S6). The interaction with the surface of MoS2 (S


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atoms) also shows similar values: Li-S and H-S binding energies are calculated as 1.3 and 0.6 eV, respectively (Fig. S6). Based on the theoretical results, the difference between Li and H atoms is not much on MoS2 surface in terms of binding energy. In addition, in previous reports, deep-level traps which contribute to non-radiative recombination process were observed for both Li and H absorbed S-vacancies in MoS2.26, 35 A recent theoretical paper suggested passivation of sulfur vacancies by three H atoms.36 Our results follow the scenario, but the formation energy was high, therefore further considerations are still required.

According to the above discussion,

the protonation on MoS2 itself seems to be not a special characteristic, but rather the H atoms would induce a modification of the surface26 in some way and inactivate the defect sites to change the optical properties of MoS2. Key factors for the successful acid treatment are summarized in Figure 5. The solvent used is important for tuning the acidity (pKa) of the solution. Molecular acids (such as TFSI) are suitable in terms of using organic solvents (DCE, etc.) to increase the acidity, but strong inorganic acids, such as H2SO4, are difficult to co-use with organic solvents (H2SO4 reacts violently with them). The types of anion moiety of acids are also important, as the dissociation constant of protons greatly depends on the tendency of solvation of the anion moiety, meaning the anion moiety is also an adjustment factor for acidity (pKa). From the viewpoint of water as a solvent, strong acids (TFSI and H2SO4 in this study) show a similar effect due to the leveling effect of the proton concentration, indicating anion dependency was less effective in this case. Finally, acidic protons (protonated solvents) are the important factor to improve the PL intensity of exfoliated monolayer MoS2.


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CONCLUSION In this study, we demonstrated the importance of the protonated environment to increase the PL intensity of monolayer MoS2. The results demonstrated that TFSI in DCE showed higher PL enhancement than that in acetonitrile and water. This difference would be due to the interaction of the solvent molecule/MoS2 and the higher acidity in DCE. Comparison of the same concentration of TFSI and H2SO4 in water showed that the PL enhancement was similar, indicating that the protonated environment is more important than the types of anion moiety. Further consideration was made with Li2SO4 in water, with and without adding H2SO4. It was further revealed that only the addition of the acid enhances PL intensity, and the protonated environment was important for the enhancement. Although the acid treatment involves a charge transfer process indicated from Raman spectra, it is not entirely concluded, but the treatment may include inactivation of a non-radiation process indicated from the analyses of PL spectra. The results would be useful to solve the mechanism to increase the PL intensity via superacid treatment and to improve the optical quality of TMDC for the demonstration of the future atomically thin optoelectronic devices.


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FIGURES

Figure 1. Schematic illustration of the acid treatment on a monolayer MoS2. (a) Dissociation of acids in a solvent into a protonated solvent and anion moiety. The anion moiety should be solvated (it is omitted for clarity). (b) The experimental process for the acid treatment. An exfoliated MoS2 was treated with an acid solution followed by annealing. The sample was evaluated with PL measurement.


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Figure 2. (a) Schematic process of the deprotonation of TFSI molecules in a solvent (S). PL spectra for the TFSI molecular treatment in (b) DCE, (c) acetonitrile and (d) water. Red and blue curves are the spectra for just exfoliated and the TFSI treated samples, respectively.


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Figure 3. PL spectra for (a) H2SO4 treated and (b) water treated monolayer MoS2. Just exfoliated samples (red curve) were treated with (a) H2SO4 solution or (b) water followed by annealing from 10 to 40 minutes (gray to blue curves). (c) Peak intensity in the spectra of H2SO4 treated (in Fig.3a) and water treated (in Fig.3b) as a function of annealing time. (d) Raman spectra for (bottom) just exfoliated and (top) H2SO4-treated (40 minutes annealed) monolayer MoS2. E’ is MoS2 in-plane mode and A’ is MoS2 out-of-plane mode. (e) PL spectra for (bottom) just


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exfoliated and (top) H2SO4-treated (40 minutes annealed) monolayer MoS2. The gray curve is obtained via subtraction of the background. Black curves are the fitted line for the gray curves. Blue, red and green curves are deconvoluted spectra for bound exciton (XA-), exciton A (XA), and exciton B curves, respectively. (f) Integrated intensity (area of the spectra in Fig. 3e) for the XA and XA- as a function of annealing time. (g) The ratio of the integrated intensities of the XA and XA- in Fig.3f.

Figure 4. (a) Schematic illustration of the interaction between MoS2 and cation moieties (protonated solvents in acid solutions). (b, c) PL spectra for a just exfoliated monolayer MoS2


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(red curves) and treated with aqueous solutions of (b) Li2SO4 or (c) a mixture of Li2SO4 and H2SO4 followed by annealing from 10 to 40 minutes (gray to blue curves).

Figure 5. Illustrative image for the key components in acid treatment for monolayer MoS2. Solvent (S) molecules tune the acidity (pKa). Anion moiety solvated in a solution is also a component for determining acidity (pKa). The protonated solvent is a key PL enhancement factor for the exfoliated monolayer MoS2 from this study.


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TABLE Table 1. The list of chemicals and solvents treated on monolayer MoS2

ASSOCIATED CONTENT Supporting Information The Supporting Information is available. DFT calculation details, deconvoluted PL spectra for H2SO4 treated, Raman spectra for just exfoliated and H2SO4 treated, binding energies of Li-S defect, H-S defect, Li-S, and H-S, an illustration of three-level system.

AUTHOR INFORMATION Corresponding Author


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*Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Sample preparation chemicals were funded by JSPS KAKENHI Grant Number 16H07127. The characterization of the optical properties was supported by JST PRESTO.


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SYNOPSIS


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Systematic Study of Photoluminescence Enhancement in Monolayer

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