pH-Dependent Photoluminescence Properties of Monolayer Transition

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C: Physical Processes in Nanomaterials and Nanostructures

pH-Dependent Photoluminescence Properties of Monolayer Transition-Metal Dichalcogenides Immersed in an Aqueous Solution Wenjin Zhang, Kazunari Matsuda, and Yuhei Miyauchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03427 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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The Journal of Physical Chemistry

pH-Dependent Photoluminescence Properties of Monolayer Transition-Metal Dichalcogenides Immersed in an Aqueous Solution Wenjin Zhang, Kazunari Matsuda, and Yuhei Miyauchi* Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The physical properties of atomically thin transition-metal dichalcogenides (TMDCs) such as monolayer (1L) molybdenum disulfide (MoS2) are easily affected by various surface interactions because of their large specific surface area. Here, we report the effects of electrochemical interactions at the TMDC–water interface on the optical properties of 1L-MoS2 using in situ photoluminescence (PL) spectroscopy in an aqueous solution whose pH was varied. The PL intensity substantially increased after exfoliated 1L-MoS2 was immersed into distilled water. Moreover, considerable modulations of the PL intensity and decay time in response to changes in the pH of solution were observed. The enhancement of the PL intensity in water and its pHdependent modulation are attributed to the carrier extraction or injection via electrochemical reactions at the interfaces between 1L-MoS2 and an aqueous solution depending on the redox potential determined by the solution pH as well as the defect passivation effect under low-pH conditions.


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INTRODUCTION Monolayers (1L) of transition-metal dichalcogenides (TMDCs) MX2, where M and X are a transition-metal (typically Mo, W) and a chalcogen (typically S, Se, or Te), have recently attracted a great deal of attention as a new class of two-dimensional (2D) direct-gap semiconductors1,2. Because of the striking change in their electronic structures from indirect (>2L) to direct bandgap (1L) depending on the number of layers1,3,4, 1L-TMDCs show strong potential for use in future electronic and optoelectronic applications such as phototransistors5–11, light-emitting devices12, and solar cells13, which rely on their excellent optical and electrical properties. Recently, modulations of the photoluminescence (PL) intensity and spectra of 1LTMDCs were demonstrated using various methods, including chemisorption and physisorption doping14–17, covalent bonding18,19, and electrostatic methods20,21. The electronic properties of 1LTMDCs under chemical doping conditions were also studied22. These studies have shown that the modulations in the PL and electronic properties were enabled by electron extraction from or injection into 1L-TMDCs via surface charge transfer phenomena15 and/or by defect passivation effects19,23. To date, most of these intriguing optical properties have been intensively studied using optical spectroscopy techniques under vacuum or ambient air conditions2,18–28. Although pioneering works24,29 have reported PL modulation effects in various solvents, knowledge of the optical properties of 1L-TMDCs in aqueous solutions with various pH values is still limited. Because various electrochemical phenomena occurring at the interfaces of water and atomically thin TMDCs can substantially modify their physical/chemical properties and affect their usefulness in applications, clarifying the influence of the surface electrochemical phenomena in water on their optical properties is important.



In the present paper, we present optical spectroscopic studies on mechanically exfoliated 1LMoS2 immersed in water. For 1L-MoS2, exciton PL intensities increased after the sample was immersed into distilled water and the spectral shape was considerably modified. The spectral analysis suggests that the exciton PL increased and the charged exciton (trion) PL decreased after the immersion. We also examined the pH dependence of the PL properties. The results suggest that the tunability of the PL is mainly attributable to charge transfer between 1L-MoS2 and an O2/H2O redox system. Moreover, we found that the PL spectral shape shows no meaningful change when the pH is less than 7, whereas the PL intensities and effective lifetimes continue to increase as the pH is decreased below pH ~7. These observations imply that not only the carrier density modulation but also passivation of local negatively charged defects can cause these PL intensity and lifetime enhancements under low-pH conditions.

EXPERIMENTAL METHODS Sample preparation. The MoS2 sample was mechanically exfoliated30 from bulk crystal and subsequently transferred onto a dimethylpolysiloxane (PDMS) film. Monolayer MoS2 was identified by optical contrast using an optical microscope, and a 2.330 eV (532 nm) cw laser was used to get the Raman and PL spectra, which were used to confirm that the sample was a monolayer31 (See Figure S1 in Supporting Information). Finally, the monolayer MoS2 sample was transferred onto a transparent glass substrate. Figure 1a shows the optical image of MoS2 on a glass substrate. The region indicated by green dashed lines corresponds to the monolayer. A glass tube was bonded to the substrate to compose a liquid reservoir for optical measurements of samples under aqueous solution conditions.


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Optical measurement. The optical measurements were performed under a home-made confocal optical measurement system with an excitation wavelength of 2.138 eV (580 nm) and the schematic is shown in Figure 1b. A spectrograph equipped with a liquid-nitrogen-cooled CCD detector was used to obtain the PL spectra. An avalanche Si photodiode and a time-correlated single-photon counting technique with pulsed excitation of ~20 ps and 40 MHz in pulse duration and frequency were used for photon detection. PL spectra were first collected with the sample in ambient air and then in distilled water. The pH of the aqueous solution was modulated using small amounts of NaOH and H2SO4 solution as pH adjustors. PL spectra and time-resolved decay profiles were then recorded in an aqueous solution with various pH. The amount of aqueous solution was kept the same for each measurement, and all of the measurements except for the initial characterization (see Supporting Information) were performed under 2.138 eV (580 nm) laser excitation conditions (excitation power density of ~500 mW/cm2) and 1.95 eV (635 nm) edge filter was used to cut the incident light for the detection. The PL spectra and timeresolved decay profiles were measured under ambient air, in distilled water, and finally in aqueous solution under various pH conditions.

Figure 1. (a) Optical image of 1L-MoS2 on a transparent glass substrate. The region surrounded by green dashed lines corresponds to the monolayer part. (b) Schematic of the optical measurement setup.



RESULSTS AND DISCUSSION Figure 2a compares the PL spectra of 1L-MoS2 in ambient air (black curve) and in water (red curve). The PL intensity of 1L-MoS2 increased dramatically after the sample was immersed into water. Considerable spectral shape change was also observed from the comparison shown in Figure 2b. To understand this change, the PL spectra were decomposed into exciton (X, ~1.90 eV) and charged exciton (trion) (X−, ~1.85 eV) peaks15,24,29,32 by a peak-fitting procedure; the results are shown in Figures 2c and 2d. The fitting results clearly suggest that the negative trion is dominant in the spectrum of as-prepared 1L-MoS2 that is normally heavily n-doped, consistent with previous reports15,20,33. The PL intensity ratio between the exciton (IX) and trion (IX−) peaks (IX/IX−) considerably increased after the sample was immersed into distilled water; this observation suggests that the trion formation was suppressed. Thus, we infer that the charge density in the 1L-MoS2 was reduced after the water immersion.

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Figure 2. (a) PL spectra of 1L-MoS2 in water (red curve) and in ambient air (black curve). (b) The PL spectra in (a), normalized for comparison. (c) and (d) Spectral fitting of the PL spectra shown in (a) with the exciton (X) and trion (X−) peaks.

We next examined the PL spectral changes of 1L-MoS2 in aqueous solution as the pH value was varied. The pH of the solution was first controlled in the acidic range and was then increased to the basic range using H2SO4 and NaOH solutions as pH adjusters. We also confirmed that the observed pH-dependent change was reproduced when the pH was again decreased by the addition of H2SO4 solution; thus, the observed PL intensity decrease is not attributable to photoinduced degradation. Figures 3a and 3b show the observed PL spectra in aqueous solution with (a) increasing and (b) decreasing pH. In both cases, the PL intensity was high when the pH of the aqueous solution was low. Figure 3c shows the integrated PL intensity as a function of the pH value of the aqueous solution. As the pH was increased (decreased) from ~3 (12) to 12 (3), the integrated PL intensity decreased (increased) by more than one order of magnitude. Figure 3d compares the PL spectra under representative pH conditions. The PL spectral shapes corresponding to pH > 8 are qualitatively different from those corresponding to pH < 8. When the pH value is smaller than pH ~7, the exciton PL dominates the spectra. By contrast, the exciton PL intensities were greatly reduced for pH > 8, and the PL intensity ratio between the exciton (IX) and trion (IX−) peaks (IX /IX−) dramatically decreased; this result suggests that the carrier density was considerably modulated depending on the pH of the aqueous solution. From the dependence of the changes in exciton and trion intensity ratio depending on the pH of the aqueous solution, we now roughly evaluate the magnitude of the carrier density modulation. According to the mass action law associated with trion, the population of exciton and trion



are related to electron density nel as

=

ℏ exp − , where , , and

are the effective mass of a trion, an exciton and an electron, respectively, is the Boltzmann constant, is the temperature, is the binding energy of trions15,34. With an assumption that the effective radiative decay rate of an exciton is about one order of magnitude larger than that of a trion15,35, nel is evaluated to be on the order of 1012 cm−2 (at pH < ~7) and 1013 cm−2 (at pH > ~8). Thus, at pH < ~7, the rate of exciton–carrier collision that leads to trion formation is reduced and the decreased nonradiative relaxation of excitons into trions can result in an enhancement of the PL intensity under the conditions pH < ~7. (b)

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0 1.7


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Figure 3. (a, b) PL spectra of 1L-MoS2 under various pH conditions observed with (a) increasing and (b) decreasing pH. (c) Integrated PL intensity as functions of pH corresponding to (a) (red) and (b) (black). (d) Comparison of normalized PL spectra for typical pH values.


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To better understand the effect of the aqueous environment on the PL behavior of 1L-MoS2, we used the Marcus–Gerischer (MG) model36 to explore the possible mechanism of the observed spectral change (Figure 4). The MG theory has been applied to explain charge transfer between a semiconductor (or metal) and a redox system through electrochemical reaction in previous reports14,37–42. At room temperature and under ambient air, some amount of oxygen molecules dissolves into water. After the sample has been immersed, charge transfer may occur from 1LMoS2 into the O2/H2O redox system as O! "#$ + 4H ( + 4) * "MoS! $ ⇌ 2H! O under slightly acidic conditions. According to the MG theory, molecules or ions in the aqueous solution have discrete energy levels that fluctuate with time37. When the Fermi energy or chemical potential of 1L-MoS2 is higher than the redox potential of the redox system, electrons could transfer from the occupied states of the sample to the unoccupied states of the O2/H2O redox system through electrochemical reaction, as shown in Figure 4(a). The redox system is regarded as the electron acceptor or donor depending on the electron transfer direction determined by the relative order between the material’s chemical potential and the redox potential. According to the Nernst equation, the redox potential of the O2/H2O redox system can be determined by electrochemical redox couples involving dissolved oxygen in aqueous solution as the relation shown below (slightly acidic conditions)37,41: 8 01234 = 5"67$ + 01234 −

2.303

where 5"67$ is the electrochemical potential of an electron in the standard hydrogen 8 electrode (SHE) relative to the vacuum level (approximately −4.44 V), 01234 is the standard

electrode potential of the reaction versus SHE in vacuum level (−1.229 V and −0.401 V under acidic and basic conditions, respectively),

!.H8HI J

=

8.8KL

V, and CD is the O2 partial pressure at



room temperature37,41. Through this approach, modulation of the redox potential is related to the variation in the pH of the solution.

Figure 4. Electron transfer mechanism within the Marcus–Gerischer (MG) model: (a) At lowpH. (b) At high pH. Left, density of states (DOS) around the band edge of MoS2 versus vacuum level. The conduction and valance bands with a direct band gap of ~1.9 eV are assumed. Right, DOS of the O2/H2O redox system. eEredox is the energy of solution at which oxidizing and reducing species are equal, Dox = Dred, where D is the DOS. eEox and eEred are mean energies for unoccupied and occupied states, respectively. The red (a) solid and (b) dashed arrows indicate the direction of charge transfer.

According to previous reports18,43–47, the conduction-band minimum (CBM) is near −4.1 eV and the valance-band maximum (VBM) is near −6.0 eV. At pH = 6.5, eEredox is estimated to be −5.27 eV. Thus, after the 1L-MoS2 has been immersed into water, electron transfer occurs from the 1L-MoS2 to the O2/H2O redox system through electrochemical processes because of the mismatch between the chemical potential of the 1L-MoS2 and the redox potential of the surrounding redox system until equilibrium is reached (Figure 4). For the distilled water, which was at pH 6.5, the redox potential lies below the Fermi energy of the 1L-MoS248, which provides


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a driving force for the electron transfer from the MoS2 to the water. Thus, trion formation is suppressed and the exciton PL increases as shown in Figure 3. When the pH is tuned to the lower side (Figure 4a), the redox potential of the solution is modulated within the range from −5.27 eV (pH 6.5) to −5.66 eV (pH 0). Because the driving force for the electron transfer from 1L-MoS2 to the solution should increase with increasing difference in electrochemical potential between the sample and the aqueous solution, under low-pH conditions, the carrier density in the 1L-MoS2 is further reduced and trion formation is further suppressed; this condition results in the enhanced exciton PL intensity observed in Figures 3 and 4. By contrast, under basic conditions (high pH), the electrochemical potential of the aqueous solution increases in the range from −5.27 eV (pH 6.5) to −4.83 eV (pH 14), which may result in suppression of the electron transfer from the 1LMoS2 to the aqueous solution, as shown in Figure 4b. This suppression leads to a relatively high electron density, and the trion formation rate increases (thus, the exciton PL intensity gradually decreases) with increasing pH. To further confirm the above mechanism, we also observed the PL spectral change under argon gas bubbling to remove the dissolved oxygen. The results also support that the O2/H2O redox couple is responsible for the observed phenomena (see Figures S2 and S3 in Supporting Information). Although the changes in spectral line shape with increasing or decreasing pH can be understood as discussed above, we also noticed in Figure 3d that the PL spectral shapes are almost unchanged for pH values less than 7; by contrast, the PL intensity is enhanced with decreasing pH, as shown in Figure 3c. This behavior implies that the PL intensity enhancement under low-pH conditions may not be attributable solely to the reduced exciton decay rate into trion states due to the decreased doped carrier density. To further examine the origin of the pHdependent PL intensity modulation, we measured time-resolved PL decay profiles under various



pH conditions, as shown in Figures 5a and 5b. We observed that the PL decay time became shorter (longer) when the pH value increased (decreased). Figure 5c plots the effective lifetime of excitons as functions of the pH, which is defined as a parameter being proportional to the exciton PL quantum yield and obtained by the fitting procedure using convolutions of biexponential function I(t) = C1exp(−t/τ1) + C2exp(−t/τ2) and the instrumental response function (IRF). The was calculated as = (C1τ1 + C2τ2)/(C1 + C2). The changes in are similar to the changes in the PL intensity shown in Figure 3. Figure 5d plots as a function of the observed PL intensity for pH ≤ 7 shown in Figure 3. The and the integrated PL intensity show a clear first-order correlation with each other (dashed line); this result suggests that the observed PL intensity change under low-pH conditions is mainly attributable to the changes in the PL decay time. Because the PL quantum yield of the as-prepared 1L-MoS2 obtained by the mechanical exfoliation method is normally very low23,49, the exciton PL decay is dominated by non-radiative relaxation pathways in the 1L-MoS2. Therefore, these results suggest that the modulation of the pH strongly affects the non-radiative relaxation processes in 1L-MoS2. The enhancement of the PL lifetimes under pH ~7 with almost no change in the PL spectral line shape (thus the negligible change in the ratio IX/IX−) implies that the nonradiative recombination rate of excitons was reduced even when the trion formation rate was unchanged. This reduction of the nonradiative recombination rate of excitons may arise from the negatively charged local defects being passivated21,50, similar to the previously reported PL brightening due to defect passivation by the super acid treatment of 1L-MoS223. The defect energy level of a sulfur vacancy under the conduction-band edge of approximately 0.45–0.7 eV50–52 may be partially passivated by the H+ ions that are more abundant under lower-pH conditions.


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Figure 5. (a, b) PL decay profiles of 1L-MoS2 under various pH conditions, as observed with (a) increasing and (b) decreasing pH. (c) Effective PL lifetimes as functions of pH for the measurements with (a) increasing (red) and (b) decreasing (black) pH. (d) The effective PL lifetimes (same data in (c)) plotted as functions of the integrated PL intensities at corresponding pH conditions (data shown in Figure 3c).

CONCLUSIONS In summary, we studied the PL properties of 1L-MoS2 in an aqueous solution at various pH levels. The PL properties of mechanically exfoliated 1L-MoS2 were considerably varied after the immersion into water, and the modulation of the solution pH further affected the PL intensity and decay time. Both the PL intensity and decay time gradually increased (decreased) as the pH of the aqueous solution decreased (increased). Modulation of the PL intensity was explained with consideration of the expected electrochemical reaction at the interface of a 1L-MoS2



semiconductor and an O2/H2O redox system under the various electrochemical potential conditions tuned by the solution pH. The analysis of the pH-dependent PL intensity and lifetime change implied that passivation of negatively charged local defects with H+ ions may also contribute to the PL enhancement under low-pH conditions. Our results may provide new insights into how the electrochemical phenomena at the interface of the atomically thin semiconductors and oxygen/water redox system affect their optical properties, the effects of which are unavoidable when the monolayer material is in a practical surrounding with finite humidity or in water.

ASSOCIATED CONTENT Supporting Information PL and Raman spectra of 1L-MoS2; comparison of PL spectra of 1L-MoS2 in air, distilled water and degassed water (PDF). This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This research was supported by JSPS KAKENHI Grant Numbers JP16H00911, JP15K13337, JP15H05408, JP15F15313, JP26390007, JP26107522, JP25400324, JP15K13500, JP16H00910, JP16H06331, JP17K19055, by the Asahi Glass Foundation, by JST CREST (JPMJCR16F3), by the Murata Science Foundation, by the Research Foundation for Opto-Science and Technology, and by the Nakatani Foundation.



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TOC Graphic


pH-Dependent Photoluminescence Properties of Monolayer Transition

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