Exciton Polarization and Renormalization Effect for Optical Modulation

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Exciton Polarization and Renormalization Effect for Optical Modulation in Monolayer Semiconductors Jiang Pu, Keichiro Matsuki, Leiqiang Chu, Yu Kobayashi, Shogo Sasaki, Yasumitsu Miyata, Goki Eda, and Taishi Takenobu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03563 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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Exciton Polarization and Renormalization Modulation in Monolayer Semiconductors

Effect

for

Optical

Jiang Pu1*, Keichiro Matsuki2, Leiqiang Chu3,4, Yu Kobayashi5, Shogo Sasaki5, Yasumitsu Miyata5, Goki Eda3,4,6, and Taishi Takenobu1,2* 1Department

of Applied Physics, Nagoya University, Nagoya 464-8603, Japan

2Department

of Advanced Science and Engineering, Waseda University, Tokyo 169-8555,

Japan 3Department 4Centre

of Physics, National University of Singapore, Singapore 117551, Singapore

for Advanced 2D Materials, Singapore 117542, Singapore

5Department

of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan

6Department

of Chemistry, National University of Singapore, Singapore 117542, Singapore

*E-mail: jiang.pu @nagoya-u.jp, [email protected]

ABSTRACT The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double layer capacitors

on

TMDC/graphite

heterostructures

to

investigate

electric-field-

and

carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic redshift of ~ 1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger redshift up to 10 meV with the accumulated carrier density above 1013 cm-2. These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced bandgap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced redshifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors. KEYWORDS: optical modulation, Stark effect, bandgap renormalization, transition metal dichalcogenides, electric double layers, photoluminescence, electroluminescence

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Monolayer semiconductors, such as transition metal dichalcogenides (TMDCs), are intrinsically lacking any dangling bonds with a thickness of below 1 nm, which thus offers one of the thinnest quantum well structures involved in high structural and chemical stability.1,2 Owing to the strong quantum confinement and the suppressed dielectric screening, the optical phenomena in these materials are governed by enhanced many-body interactions.3 It is evident from the enormous binding energy of excitons (and trions), the bounded states of electrons and holes (further coupled with additional electrons/holes), which have been revealed to be a few hundreds of meV (a few tens of meV for trions).4-6 These features outperform approximately one order of magnitude greater than those of traditional quantum wells based on III-V semiconductors, and thus, monolayer TMDCs can be ideal platforms to explore optical device applications in purely two dimensions.7,8 In particular, the optical modulation is one of the most prominent signatures in monolayer TMDCs; their optical responses can be crucially tuned by the external electric field and carrier concentrations.9 For example, the quantum-confined Stark effect (QCSE) has been recently demonstrated.10-14 In principle, the exciton polarizability against the out-of-plane electric field should be very small in monolayer TMDCs because of the tight confinement along the vertical direction (Fig. 1a). In fact, the experimentally reported Stark shifts of photoluminescence (PL) were purely up to 1 meV under an out-of-plane electrostatic field of > 1 MV cm-1.14 This result means that the intense electric field is indispensable for yielding optical modulation in conventional QCSE. Another key approach is the carrier doping tuned optical transitions, which is mainly derived from an interplay between the decrease in binding energy of the excitons and bandgap renormalization effects through electron-electron and/or electron-hole interactions (Fig. 1b).15-20 The former is caused by doping-induced dielectric screening and results in blueshifts of optical transitions.15,16 In contrast, the latter commonly generates the reduction of an electronic energy gap, which means redshifts for optical transitions.17-20 Thereby, the competition of these tunable many-body effects dominates the optical modulations under carrier accumulations. Notably, the remarkable bandgap renormalization has been expected at high carrier densities of 1013 - 1014 cm-2,21-24 in such a way that in these regimes, the renormalization effects should play the main role for generating significant optical tuning.20 To reveal the doping effects in optical transitions, the continuous and wide-range control of the material carrier density is necessary. However, such carrier density regimes are difficult to achieve for conventional techniques adopted in TMDCs, such as using an oxide-based transistor and chemical doping; consequently, the electrically tuned optical behaviors at high-carrier-density regimes have not yet been fully clarified.4,16 To investigate the electrical modulation of excitonic luminescence properties, here, we


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combined high-quality chemical vapor deposition (CVD) growth of monolayer TMDCs with electrolyte-gating methods to evaluate electric-field- and carrier-density-dependent PL properties. First, we fabricated electric double layer (EDL) capacitors with monolayer WS2 and MoS2 synthesized on graphite substrates. We initially observed the slight redshifts of exciton resonance of ~ 1 meV without carrier accumulations, which is dominantly caused by the QCSE. Interestingly, larger redshifts up to 10 meV are obtained followed by carrier accumulations that exceed 1013 cm-2. These observations represent that the optical transitions in S- and Se-based TMDCs can be significantly modulated by the carrier density control, thus arising from the primal contributions of the doping-induced bandgap renormalization effect. To stimulate further modulation capability, second, we introduced electrolyte-based light-emitting devices to evaluate tunable electroluminescence (EL) properties. Biasing solely to electrolyte-induced p-i-n junctions, we can achieve pronounced redshifts of up to 40 meV for exciton and trion EL. Consequently, our approach reveals that the doping effects in high-carrier-density regimes are important for yielding efficient optical modulation in monolayer TMDCs. RESULTS AND DISCUSSION EDL capacitors with TMDC/graphite heterostructures. To investigate the effects in optical transitions of TMDCs with inducing out-of-plane electric fields and carrier accumulations, we prepared CVD-grown WS2 and MoS2 monolayers on graphite substrates (Figs. 2a and 2b). Since monolayer TMDCs have a large binding energy that substantially exceeds the thermal activation energy, such strong confinement allows the stable coexistence of excitons and trions at room temperature.4-6 This circumstance leads primarily to the asymmetric features in the measured PL spectrum, which is the mixed contribution of excitons and trions in the

spectrum

weight.

Therefore,

the

initially

reported

Stark

spectroscopy

or

electrostatically/chemically doped optical spectroscopy required spectrum separation into two spectra through curve fitting,4,14,16 e.g., excitons and trions, or performing PL measurements at low temperature to trace their individual behavior against the electric fields and the carrier accumulations.12 The notable advantage to introducing the CVD-grown monolayer samples on graphite substrates is the absence of trion PL. Indeed, the previously reported PL for the TMDC/graphite heterostructures has exhibited sharp and symmetric spectrum features.25 This was interpreted by the fast non-radiative recombination process due to the energy transfer from TMDCs to graphite, which attributes to the reduction of trion contributions in whole spectrum weight.25 As shown in Fig. 2c, the measured PL spectra of monolayer WS2 grown on a graphite can be perfectly fitted by a single Lorentzian function with narrow full width at a half maximum (FWHM) of 20 meV, in such a way that it allows us to evaluate a pure exciton response at room temperature (also see Supporting Information S1 for MoS2). Furthermore, in this


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TMDC/graphite heterostructure, the graphite substrates can serve as one electrode of a capacitor configuration to break the electric field screening in the TMDC layers. To complete the fabrication of an EDL capacitor, we deposited another gold electrode separated from the samples, followed by spin-coating thin ion-gel films, the gelation of an ionic liquid by a polymer with a thickness of below 100 nm, on the surface of the TMDC/graphite heterostructure and electrode (Fig. 2a).26 When the voltage is applied between the graphite and electrode, the anions and cations are redistributed to each TMDC and electrode surface to form the EDLs. The thickness of the EDLs is ideally down to the ion radius of 1 nm, which results in induced vertical electric fields above the order of MV cm-1 into the TMDC/graphite heterostructures. Moreover, the huge specific capacitance of the EDLs (~ 10 μF cm-2) enables the continuous control of the sheet carrier density, n2D, up to 1014 cm-2; thus, adopting the EDL capacitors is beneficial for the wide-range evaluations of both electric-field- and carrier-density-dependent optical transitions.27,28 Electrical modulation of exciton PL. Figure 2d shows the applied voltage dependence of the PL spectra for monolayer WS2 in the EDL capacitor, when recorded at room temperature. The measured PL spectra initially show a slight redshift, following by exhibiting a larger redshift and a decrease in the emission intensity, without compromising the symmetric spectrum shape. To further visualize the PL behaviors against the applied voltage, Figure 2e indicates the 2D color mapping of the obtained PL spectra as a function of the applied voltage and the photon energy. The emission energy is nonlinearly shifted to lower energy with increasing voltage applications. We also measured the voltage-dependent PL spectra for monolayer MoS2, and similar peak energy redshifts and intensity variations were obtained (Fig. S1). Therefore, we performed the spectrum analysis by a single Lorentzian function for both measured PL in WS2 and MoS2 to clarify the detailed exciton behaviors. Figures 3a, 3b, and 3c exhibit the reference voltage, VR, dependence of the exciton peak energy, the integrated PL intensity, and the FWHM, respectively, extracted from the peak fitting for WS2. Two results for different devices are shown to confirm the reproducible performances in our measurements. It should be noted that the applied voltage is consumed in both interfaces of electrodes and TMDC/graphite heterostructures because two EDL capacitors are connected in series, as shown in Fig. 1a. For estimating the effective voltage contribution on the TMDC interface, we inserted a reference electrode and measured its precise potential drop (Supporting Information S2).29 In addition to the redshifts of the exciton PL, we noticed that the redshift behaviors against the applied voltage can be further divided to two regimes: one shows a very small redshift of peak energy without any variations in the emission intensity and an FWHM under lower VR, and the other exhibits larger redshifts with decreasing emission intensity and spectral broadening at higher VR (the gray shaded area depicted in Figs. 3a, 3b, and 3c). Interestingly, the former slight


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redshift regime within a voltage range of ~ 0.35 V suggests a linear response to the voltage square. Furthermore, the absence of changes in the emission intensity and the FWHM represents no carrier doping effects in this regime, which indicates pure electric-field tuning optical transitions. On the other hand, the larger redshift of up to 10 meV against the voltage application of 0.35 V < VR < 1 V reflects carrier accumulation effects because of the greater variations in both the emission intensity and the FWHM. Thereby, we should consider the distinct optical tuning mechanisms that occurred in these two regimes. Because the measured PL spectra in all voltage regimes can be analyzed by single Lorentzian fitting without any asymmetric broadening, the gained PL behaviors are attributed to exciton contributions. The electric-field-dependent optical transitions. Since the carrier doping generally affects the PL properties, e.g., the PL intensity decreases and its spectrum broadens with increasing carrier concentrations, it is crucial to classify those influences when evaluating the intrinsic effects in an electric field and the carrier density dependence of optical modulation. In this regard, we can determine that the small quadratic redshifts in the lower voltage regime (VR < 0.35 V in Fig. 3a) are pristine exciton polarization without the variations in the carrier density in semiconductors. It should be noted that only a slight change in the PL intensity can be explained by the reduction in the overlap between the electron and the hole wave functions. As a result, the quadratic redshifts of the exciton resonance indicate a second order perturbation in the exciton recombination that originates from the QCSE.7,8 The energy shifts in the QCSE are simply written as ∆E = –μZFZ –βZFZ2, where FZ is the DC electric field, μZ is the exciton dipole moment, and βZ is the exciton polarizability along the vertical direction. It is noted that the μZ should be zero for 1s exciton, thus, the ∆E is expected to be quadratic against the FZ. To examine the exciton polarizability, it is required to convert the applied voltage to the electric field. However, the exact thickness of the EDLs has not yet been fully revealed. Some theoretical and experimental investigations have reported that the thickness of the EDLs is intrinsically down to 1 nm (depending on the ion radius).30,31 Here, by taking relevant experimental results in the ionic liquids,31 we assumed that the thickness of the EDLs is 1 nm to calculate the electric field. It is also advantageous to use graphite substrates as an electrode of EDL capacitors, and this approach can uniformly suppress the electric field screening, thus allowing linear field distributions inside the TMDCs.28 On the basis of these approximations, Figure 3d displays the electric field dependence of the exciton peak energy in the range of VR from 0 V to 0.35 V, as shown in Fig. 3a. The Fz is derived by the equation FZ = VR / d, where the d = 1.8 nm is the sum of the EDL (1 nm) and monolayer (0.8 nm) thicknesses.25 The exciton polarizability can be calculated to 1.0 x 10-20 eV m2 V-2 (4.8 x 10-10 D m V-1) for the monolayer WS2 through the parabolic fitting of Fig. 3d, which is equivalent to a linear fitting of same data as a function of the squared electric field, as shown in the inset of Fig. 3d. These values are


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almost consistent with the reported out-of-plane polarizabilities derived from both theoretical calculations and experimental results in monolayer MoS2.10,14 The polarizability difference between the reported value for MoS2 (~ 7.0 x 10-10 D m V-1) and our results of WS2 can be reflected in the difference of the out-of-plane effective mass and/or the width of the quantum well, i.e., the thickness of the monolayers is dependent on the out-of-plane lattice constant. The substrate effects would also provide substantial causes because the confinement potential depending on the stacked materials possibly changes the stability of the excitons and affects its polarizability.10 It is noteworthy that the obtained polarizability appears to be explained by the classical infinite-barrier quantum well model, where the free electron and hole masses are confined inside infinite square barriers (Supporting Information S3).10,32 This simplistic mechanism preserves the Stark shift and is proportional to the fourth power of the quantum well width (the monolayer thickness). Therefore, the obtained polarizabilities in the monolayer TMDCs are few orders of magnitude smaller than those of conventional quantum wells based on III-V materials.8 This finding means that inducing an enormous electric field is necessary for monolayer TMDCs to achieve pronounced optical modulations via a conventional QCSE mechanism; as a result, it lacks practical utility in terms of device applications. In spite of the poor sensitivity of the exciton polarization, as shown by the gray symbols in Fig. 3d, we can see that the peak shift behaviors depart from quadratic functions and show larger redshifts compared with applying higher VR, which means that distinct roles dominate the optical responses at those voltage regimes. The carrier-density-dependent optical transitions. Although the QCSE offers only slight optical controllability, we observed further significant redshifts of up to 10 meV in the higher VR of > 0.35 V (Fig. 3a). In this regime, an obvious decrease in the PL intensity and a broadening of the PL spectra were obtained (Figs. 3b and 3c), which represent that the dominant contributions of electron accumulations in the WS2 affected the PL properties. Similar behaviors were also collected in MoS2, as shown in Fig. S1. In MoS2 devices, in contrast, we were not able to gain QCSE regimes because MoS2 normally shows n-type transports due to naturally doped electrons.6,33 As a result, it is difficult to completely eliminate the doping effects, and thus, we evaluated only the doping-induced optical transitions in the MoS2. To clarify the carrier accumulation effects in the optical bandgaps of monolayer TMDCs, the reduction of exciton binding energy and the bandgap renormalizations coexist, and those interplays determine the total deviations in the optical transitions (Fig. 1b).15-20 According to some experimental measurements in doped WS2 and MoS2 using transistors and chemical doping methods, blueshifts of several tens of meV have been reported in the n2D of ~ 1012 cm-2.6,16 This finding means that the reduction in the binding energy (a few hundreds of meV) conquers the bandgap renormalizations (or mostly cancels them) in the relatively lower n2D, which is also in good


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agreement with the results of theoretical calculations.15,20 In contrast, remarkable bandgap renormalization

effects

have

been

observed

through

angle-resolved

photoemission

spectroscopy.21-24 The surface properties of alkali metals onto monolayer TMDCs resulted in a large bandgap shrinkage of up to 400 meV at higher n2D > 1013 cm-2. Furthermore, a theoretical calculation predicts that the reduction in the binding energy of the excitons tends to be saturated, and then, their renormalization effects will play a main role in dominating the optical bandgaps in the high n2D.20 Considering these competitive factors, the larger optical modulations that arise from bandgap renormalizations are feasible when the n2D exceeds 1013 cm-2. In particular, we could not acquire obvious blueshift behaviors even in low voltage regimes, which also suggest the bandgap renormalizations dominantly govern the optical transitions in the measurements. For estimating the n2D in our devices, we used a simple equation, ∆n2D = C x (VR - Vth) / e, where C is the specific capacitance of the ion gels, Vth is the threshold voltage of the electron accumulations, and e is the elementary charge. Here, Vth = 0.35 V is determined by the switching point between purely electric field contributions and doping effects, which corresponds to the onset of decreasing the PL intensity and its broadening (the shaded area in Figs. 3b and 3c). To further support this calculation, we also monitored the displacement currents in the fabricated EDL capacitors (Supporting Information S4). Additionally, there is a clear current increase in the applied voltage at approximately 0.3 – 0.4 V for WS2 devices, which is attributed to doping-induced displacement currents, while the MoS2 showed a current increase from 0 V, which means doped conditions in the off state (Fig. S4). In addition, the C is taking from our previously reported value of 5.7 μF cm-2 measured in spin-coated ion gels, which is also consistent with commonly used ionic-liquid-gated TMDC transistors (5 - 10 μF cm-2).26-29 It should be noted that the electrolyte-gating methods are powerful to tune high carrier density accumulations, but it is less sensitive to evaluate optical behaviors for lower n2D of ~ 1012 cm-2 due to their high specific capacitances. Figure 3e reproduces the exciton peak energy shifts as a function of the calculated ∆n2D for WS2 and MoS2, respectively. The ∆n2D is continuously tuned up to 2.5 x 1013 cm-2, and the peak energy is monotonically redshifting against increased n2D. Since the symmetry of the PL spectrum is preserved without any other peak generations or asymmetric features in this regime (Fig. 2d), the obtained peak shifts are reflected exciton behaviors. The blue line in the inset of Fig. 3e illustrates recently reported results in terms of photoinduced optical bandgap renormalizations in the monolayer WS2.19 Those results also revealed heavy renormalization effects in optical transitions above n2D of 1013 cm-2, which are mostly in agreement with our results. The relevant experiments of optically-pumped bandgap renormalizations has also predicted the Mott density in TMDCs is around 4 - 10 x 1013 cm-2 to induce transition from


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excitonic regimes to electron-hole plasma.34,35 It should be noted that, because the photo-pumping measurements induce both electron and hole excitations, it would cause the difference of estimated carrier density compared to that of electrostatic doping. Moreover, the recent theoretical calculations also present that the exciton resonance energy prevails by bandgap renormalization effects at the orders of n2D above 1012 cm-2, while the competition between the reduction in the binding energy and the band shrinkage yields almost cancelations for optical transitions at n2D < 1012 cm-2.20 The theoretically predicted peak energy variations for monolayer MoS2 within our available carrier density regimes (n2D > 1013 cm-2) are shown as a gray line in the inset of Fig. 3e, whose behavior is qualitatively in agreement with our results and with reported experimental results. It is noted that although the energy shifts for bandgap renormalizations in semiconductor quantum wells are generally known to be proportional to n2D1/3, the obtained results show a larger exponent for the energy shifts.36 This finding could reflect more complex factors to reconcile the total optical transitions, thus offering future subjects to determine the detailed dynamics/mechanisms of tunable electronic structures via many-body effects. Electrical modulation of exciton and trion EL. To further expand the modulation capability in our approach, we attempted to demonstrate the EL modulations, which is an important operation to directly modulate the light source signal. Recently, we established a very simple electrolyte-based method to build TMDC light-emitting devices with ion gels.37,38 Using this method, we fabricated two-terminal devices followed by covering spin-coated ion-gel films onto the exfoliated WSe2 monolayers, as shown in the optical image of Fig. 4a. Because the monolayer WSe2 can easily gain ambipolar (both hole and electron) transports, it is suitable to use as an active material in light-emitting devices.29 When only biasing with a few volts between the two electrodes, the cations and anions are redistributed to form the EDLs on the opposite electrodes. These EDLs induce electrostatic doping inside WSe2, which results in the formation of p-i-n junctions to generate EL (Fig. 4a). Figure 4b presents the current-voltage characteristics measured at temperature T, which is 280 K. Typical diode-like operations were obtained, and thus, we performed EL spectroscopy against voltage applications. Figure 4c exhibits a comparison between the PL spectrum without biasing and the EL spectrum with the relatively smaller bias of 2.4 V. The perfect consistency between PL and EL preserved excitonic EL generation from a WSe2 p-i-n junction. Figure 4d indicates voltage dependent EL properties that range from 2.4 V to 4.2 V, and the inset of Fig. 4d exhibits their zoomed spectra. We can clearly observe the redshifts of the emission peak energy and the spectrum broadening, as well as the obtained behaviors in Figs. 3a and 3c. Figure 4e represents the 2D color mapping of the EL spectra as a function of the applied voltage, with the photon energy ranging from 1.5 eV to 1.75 eV. The nonlinear redshifts of up to 40 meV can also be clarified. In addition, as shown in


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the inset of Fig. 4b, the integrated EL intensity increased linearly with the current injections, which also provides the direct evidence of EL generation from the electrolyte-induced p-i-n junction.37 On the basis of these results, we performed spectrum analysis for each obtained EL spectrum (Supporting Information S5). Since the EL spectrum exhibits asymmetric features, we adopted two Voigt functions; one fitted curve can be assigned to the excitons, and the other is assigned to the trions. It is noted that there are lower energy transitions of approximately 1.2 eV derived from the bulk part of the devices and the localized states approximately 1.5 eV in the whole spectrum (Fig. 4d). Here, we mainly focused on the peak behaviors that originated from direct-gap transitions of monolayer WSe2. Curve fittings were conducted in all of the measured EL spectra, and the individual peak energy shifts for the exciton and trion EL are summarized in Fig. 4f (also, see details in Supporting Information S5). Both the exciton and trion EL are monotonically tuned to lower energy (~ 40 meV), and their behaviors are equivalent to the PL results of WS2 and MoS2, as shown in Fig. 3e. We also prepared an additional device to confirm the reproducible measurements as exhibited in Fig. 4f. In particular, we observed an opposite response for the EL intensity between the exciton component and that of trions; the exciton EL decreased, while the trion EL increased with the voltage applications (Fig. S5). Such a difference in the intensity variation between excitons and trions renders that the carrier recombinations dominantly occurred at p- and/or n-doped regions, which supports the evidence for doping effects in the measured EL.39 Therefore, we identified that the yielded EL modulations in the WSe2 devices are also mediated by doping-induced optical bandgap tuning. The n2D in the doped regimes can be estimated up to 4.0 x 1013 cm-2 by assuming simple equivalent circuits based on our previous reports,37 which causes these results to also agree with the calculated signatures shown in Fig. 3e. Although efficient EL modulation was realized in monolayer WSe2, it should be addressed that the obtained emission peak energy variations were not affected by adverse effects such as current flowing because the Joule heating possibly causes the spectrum redshifts. To reduce the influence of the heating effects, we also examined the temperature dependence of the PL and EL spectra measured at T in the range from 280 K to 80 K (Supporting Information S6). With decreasing temperature, both the PL and EL spectra exhibited a peak energy blueshift and spectrum narrowing, which are reasonably well reproduced with the Varshni equation, an equation that describes the temperature dependence of bandgap variation in semiconductors.40 Importantly, the exciton and trion peak energy shifts in EL show a similar tendency as well as that of PL at T in the range of 280 K to 160 K (Fig. S6), which is less affected by the thermal effects in those temperature regimes. In contrast, the energy shift behaviors at T < 160 K were seriously influenced by the Joule heating. The blueshift behaviors are being saturated at lower


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T; i.e., the emission resonance apparently acted larger redshift behaviors. Thereby, we can conclude that the observed redshifts of the exciton and trion EL at nearly room temperature originates from primary variations in the optical bandgap transitions due to doping effects. CONCLUSIONS In summary, we investigated electrically tunable optical transitions in monolayer TMDCs by using electrolyte-based device structures. In principle, the optical modulations in monolayer TMDCs can be divided into two schemes: one is the electric-field-induced Stark effect, and the other is the doping-induced optical gap variation mediated by many-body effects. In particular, the latter is determined by the competition between the reduction of the exciton binding energy and bandgap renormalization effects. To reveal these comprehensive schemes in the use of TMDCs for optical modulations, first, we fabricated combined EDL capacitors with TMDC/graphite heterostructures to evaluate exciton behaviors against wide-range electric-field- and carrier-density-modulated PL measurements. The exciton PL initially showed a slight quadratic redshift of ~ 1 meV without carrier accumulations, which corresponds to the dominant contribution of the QCSE. The PL emission energy further yielded redshifts of up to 10 meV at an n2D that exceeded 1013 cm-2. This finding is mainly caused by the optical bandgap renormalization effects via many-body interactions, which is in good agreement with theoretical predictions. Although the exciton of the TMDCs is polarizable, its polarizability is quite small because of the tight confinement along the vertical direction. On the other hand, we realized larger electrical controllability of optical transitions by tuning n2D, which results in alternative solutions to achieve pronounced optical modulations in TMDCs. Moreover, to further explore the modulation capability in our approach, we introduced the electrolyte-based light-emitting devices to demonstrate EL modulations in monolayer TMDCs. As a result, we obtained exciton and trion EL from electrolyte-induced p-i-n junctions, and their resonance energies were monotonically tuned to the lower energy of 40 meV. Consequently, the combination of monolayer TMDCs and electrolyte-gating methods achieved efficient optical modulations in both PL and EL. The demonstrated approaches reveal that the doping effects at high-carrier-density regimes enable significant optical modulation ability, possibly offering a pathway for developing high-performance optical and photonic devices based on monolayer semiconductors.


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METHODS Growth of monolayer TMDCs on graphite. The WS2 (MoS2) monolayers were grown by the CVD system developed by our previous study.25 In brief, the exfoliated graphite flakes onto a glass substrate, which serve as growing templates, was placed in a quartz tube with WO3 (MoO3) powders. The temperatures of the glass substrates and WO3 (MoO3) powders were gradually increased ranging from 900 to 1100 °C by the electrical furnace to facilitate sulfurization on the graphite templates. Then, the sulfur was heated at 200 °C to flow inside a quartz tube with Ar as a carrier gas (flow rate of 100 cm-2 min). After 15 – 30 mins sulfurization, the tube furnace was rapidly cooled down to room temperature. The capacitor and light-emitting device fabrications. After the growth of TMDC/graphite heterostructures, two gold electrodes are deposited on the substrate: one is attached on the graphite, and the other is separated from the samples. Then, thin ion-gel films (< 100 nm) were spin-coated onto substrates to form EDL capacitor configurations. The reference electrode was also inserted between two electrodes. For light-emitting devices, the WSe2 samples were mechanically exfoliated onto SiO2 substrates. After exfoliation, two gold electrodes were patterned by standard electron beam lithography, followed by spin-coating ion-gel films. The ion

gels

were

mixed

with

a

triblock

copolymer,

poly(styrene-block-methyl

methacrylate-block-styrene) (PS-PMMA-PS; MPS = 4.3 kg mol-1, MPMMA = 12.5 kg mol-1, Mw = 21.1

kg

mol-1),

and

an

ionic

liquid,

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), in an ethyl propionate solution for both capacitor and light-emitting device fabrications.26,37 PL and EL spectroscopy. The PL and EL measurements were performed using a micro-Raman spectroscope (Renishaw, inVia) and confocal microscope with a Raman system (NT-MDT NTEGRA), respectively. The devices were set on a cryostat with N2 flow or vacuum condition (< 10-3 Pa) to conduct combined electrical and optical measurements. The temperature of the cryostat was controlled from 300 K to 80 K with liquid nitrogen flow. In particular, the PL measurements were excited by a 532-nm laser.

ASSOCIATED CONTENT Supporting Information Available: The PL results for MoS2 devices, the evaluations of EDL capacitors, and the details for PL and EL spectrum analysis. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest.


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AUTHOR INFORMATION Corresponding Author *E-mail: jiang.pu @nagoya-u.jp, [email protected]

ACKNOWLEDGMENTS T.T. was partially supported by JSPS KAKENHI (Grant Numbers JP15K21721, JP26102012, JP25000003, and JP17H01069) and JST CREST (Grant Number JPMJCR17I5). J.P. was supported by JSPS-KAKENHI Grant Number JP17H06736. K.M. was supported by the Leading Graduate Program in Science and Engineering, Waseda University, from MEXT. Y.M. was supported by JSPS KAKENHI (Grant Numbers JP15H05412, JP16H00918, and JP18H01832) and JST CREST (Grant Number JPMJCR16F3). G.E. acknowledges the Singapore National Research Foundation and acknowledges support from the Ministry of Education, Singapore, under AcRF Tier 2 (MOE2015-T2-2-123, MOE2017-T2-1-134) and AcRF Tier 1 (R-144-000-387-114).

REFERENCES 1.

Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.;

Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl Acad. Sci. USA 2005, 102, 10451– 10453. 2.

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and

Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699–711. 3.

Xia, F.; Wang, H.; Xiao, D.; Dubey, M.; Ramasubramaniam, A. Two-Dimensional Material

Nanophotonics. Nat. Photon. 2014, 8, 899–907. 4.

Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound

Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207–211. 5.

He, K.; Kumar, N.; Zhao, L.; Wang, Z.; Mak, K. F.; Zhao, H.; Shan, J. Tightly Bound

Excitons in Monolayer WSe2. Phys. Rev. Lett. 2014, 113, 026803. 6.

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. 7.

Miller, D. A. B.; Chemla, D. S.; Damen, T. C.; Gossard, A. C.; Wiegmann, W.; Wood, T.

H.; Burrus, C. A. Band-Edge Electroabsorption in Quantum Well Structures: the


ACS Nano 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

Quantum-Confined Stark Effect. Phys. Rev. Lett. 1984, 53, 2173–2176. 8.

Kuo, Y.-H; Lee, Y. K.; Ge, Y.; Ren, S.; Roth, J. E.; Kamins, T. I.; Miller, D. A. B.; Harris,

J. S. Strong Quantum-Confined Stark Effect in Germanium Quantum-Well Structures on Silicon. Nature 2005, 437, 1334–1336. 9.

Sun, Z.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat.

Photon. 2016, 10, 227–238. 10.

Pedersen, T. G. Exciton Stark Shift and Electroabsorption in Monolayer Transition-Metal

Dichalcogenides. Phys. Rev. B 2016, 94, 125424. 11.

Scharf, B.; Frank, T.; Gmitra, M.; Fabian, J.; Zutic, I.; Perebeinos, V. Exciton Stark Effect

in MoS2 Monolayers. Phys. Rev. B 2016, 94, 245434. 12.

Klein, J.; Wierzbowski, J.; Regler, A.; Becker, J.; Heimbach, F.; Muller, K.; Kaniber, M.;

Finley, J. J. Stark Effect Spectroscopy of Mono- and Few-Layer MoS2. Nano Lett. 2016, 16, 1554–1559. 13.

Massicotte, M.; Vialla, F.; Schmidt, P.; Lundeberg, M. B.; Latini, S.; Haastrup, S.;

Danovich, M.; Davydovskaya, D.; Watanabe, K.; Taniguchi, T.; Fal’ko, V. I.; Thygesen, K. S.; Pedersen, T. G.; Koppens, F. H. L. Dissociation of Two-Dimensional Excitons in Monolayer WSe2. Nat. Commun. 2018, 9, 1633. 14.

Roch, J. G.; Leisgang, N.; Froehlicher, G.; Makk, P.; Watanabe, K.; Taniguchi, T.;

Schonenberger, C.; Warburton, R. J. Quantum-Confined Stark Effect in a MoS2 Monolayer van der Waals Heterostructure. Nano Lett. 2018, 18, 1070–1074. 15.

Gao, S.; Liang, Y.; Spataru, C. D.; Yang, Li. Dynamical Excitonic Effects in Doped

Two-Dimensional Semiconductors. Nano Lett. 2016, 16, 5568–5573. 16.

Liu, B.; Zhao, W.; Ding, Z.; Verzhbitskiy, I.; Li, L.; Lu, J.; Chen, J.; Eda, G.; Loh, K. P.

Engineering Bandgaps of Monolayer MoS2 and WS2 on Fluoropolymer Substrates by Electrostatically Tuned Many-Body Effects. Adv. Mater. 2016, 28, 6457–6464. 17.

Steinhoff, A.; Rosner, M.; Jahnke, F.; Wehling, T. O.; Gies, C. Influence of the Excited

Carriers on the Optical and Electronic Properties of MoS2. Nano Lett. 2014, 14, 3743–3748. 18.

Park, Y.; Han, S. W.; Chan, C. C. S.; Reid, B. P. L.; Taylor, R. A.; Kim, N.; Jo, Y.; Im,

H.; Kim, K. S. Interplay Between Many Body Effects and Coulomb Screening in the Optical Bandgap of Atomically Thin MoS2. Nanoscale 2017, 9, 10647–10652. 19.

Jiang, T.; Chen, R.; Zheng, X.; Xu, Z.; Tang, Y. Photo-Induced Excitonic Structure

Renormalization and Broadband Absorption in Monolayers Tungsten Disulphide. Opt. Express 2018, 26, 859–868. 20.

Meckbach, L.; Stroucken, T.; Koch, S. W. Giant Excitation Induced Bandgap

Renormalization in TMDC Monolayers. Appl. Phys. Lett. 2018, 112, 061104. 21.

Riley, J. M.; Meevasana, W.; Bawden, L.; Asakawa, M.; Takayama, T.; Eknapakul, T.;


Page 14 of 21

Page 15 of 21 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

ACS Nano

Kim, T. K.; Hoesch, M.; Mo, S.-K.; Takagi, H.; Sasagawa, T.; Bahramy, M. S.; King, P. D. C. Negative Electronic Compressibility and Tunable Spin Splitting in WSe2. Nat. Nanotechnol. 2015, 10, 1043–1047. 22.

Kang, M.; Kim, B.; Ryu, S. H.; Jung, S. W.; Kim, J.; Moreschini, L.; Jozwiak, C.;

Rotenberg, E.; Bostwick, A.; Kim, K. S. Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides. Nano Lett. 2017, 17,1610–1615. 23.

Ulstrup, S.; Cabo, A. G.; Miwa, J. A.; Riley, J. M.; Gronborg, S. S.; Johannsen, J. C.;

Cacho, C.; Alexander, O.; Chapman, R. T.; Springate, E.; Bianchi, M.; Dendzik, M.; Lauritsen, J. V.; King, P. D. C.; Hofmann, P. Ultrafast Band Structure Control of a Two-Dimensional Heterostructure. ACS Nano 2016, 10, 6315–6322. 24.

Katoch, J.; Ulstrup, S.; Koch, R. J.; Moser, S.; McCreary, M.; Singh, S.; Xu, J.; Jonker, B.

T.; Kawakami, R. K.; Bostwick, A.; Rotenberg, E.; Jozwiak, C. Giant Spin-Splitting and Gap Renormalization Driven by Trions in Single-Layer WSe2/h-BN Heterostructures. Nat. Phys. 2018, 14, 355–359. 25.

Kobayashi, Y.; Sasaki, S.; Mori, S.; Hibino, H.; Liu, Z.; Watanabe, K.; Taniguchi, T.;

Suenaga, K.; Miniwa, Y.; Miyata, Y. Growth and Optical Properties of High-Quality Monolayer WS2 on Graphite. ACS Nano 2015, 9, 4056–4063. 26.

Yomogida, Y.; Pu, J.; Shimotani, H.; Ono, S.; Hotta, S.; Iwasa, Y.; Takenobu, T.

Ambipolar Organic Single-Crystal Transistors based on Ion Gels. Adv. Mater. 2012, 24, 4392– 4397. 27.

Bisri, S. Z.; Shimizu, S.; Nakano, M.; Iwasa, Y. Endeavor of Iontronics: from

Fundamentals to Applications of Ion-Controlled Electronics. Adv. Mater. 2017, 29, 1607054. 28.

Matsuki, K.; Pu, J.; Kozawa, D.; Matsuda, K.; Li, L.-J.; Takenobu, T. Effects of

Electrolyte Gating on Photoluminescence Spectra of Large-Area WSe2 Monolayer Films. Jpn. J. Appl. Phys. 2016, 55, 06GB02. 29.

Huang, J.-K.; Pu, J.; Hsu, C.-L.; Chiu, M.-H.; Juang, Z.-Y.; Chang, Y.-H.; Chang, W.-H.;

Iwasa, Y.; Takenobu, T.; Li, L. J. Large-Area Synthesis of Highly Crystaline WSe2 Monolayers and Device Applications. ACS Nano 2014, 8, 923–930. 30.

Shimotani, H.; Asanuma, H.; Iwasa, Y. Electric Double Layer Transistors of Organic

Semiconductor Crystals in a Four-Probe Configuration. Jpn. J. Appl. Phys. 2007, 46, 3613– 3617. 31.

Yamamoto, R.; Morisaki, H.; Sakata, O.; Shimotani, H.; Yuan, H.; Iwasa, Y.; Kimura, T.;

Wakabayashi, Y. External Electric Field Dependence of the Structure of the Electric Double Layer at an Ionic Liquid/Au Interface. Appl. Phys. Lett. 2012, 101, 053122. 32.

Lukes, T.; Ringwood, G. A.; Suprapto, B. A Particle in a Box in the Presence of an

Electric Field and Applications to Disordered Systems. Physica A. 1976, 84, 421–434.


ACS Nano 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

33.

Mouri, S.; Miyauchi, Y.; Mtsuda, K. Tunable Photoluminescnece of Monolayer MoS2 via

Chemical Doping. Nano Lett. 2013, 13, 5944–5948. 34.

Chernikov, A.; Ruppert, C.; Hill, H. M.; Rogosi, A. F.; Heinz, T. F. Population Inversion

and Giant Bandgap Renormalizatio in Atomically Thin WS2 Layers. Nat. Photon. 2015, 9, 466– 470. 35.

Bataller, A. W.; Younts, R. A.; Rustagi, A.; Yu, Y.; Ardekani, H.; Kemper, A.; Cao, L.;

Gundofdu, K. Dense Electron-Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning. Nano Lett. 2019, 19, 1104–1111. 36.

Kleinman, D. A.; Miller, R. C. Band-Gap Renormalization in Semiconductor Quantum

Wells Containing Cariiers. Phys. Rev. B 1985, 32, 2266–2272. 37.

Pu, J.; Fujimoto, T.; Ohasi, Y.; Kimura, S.; Chen, C.-H.; Li, L.-J.; Sakanoue, T.;

Takenobu, T. A Verstile and Simple Approach to Generate Light Emission in Semiconductors Mediated by Electric Double Layers. Adv. Mater. 2017, 29, 1606918. 38.

Pu, J.; Takenobu, T. Monolayer Transition Metal Dichalcogenides as Light Sources. Adv.

Mater. 2018, 30, 1707627. 39.

Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus,

D. G.; Xiao, D.; Yao, W.; Xu, X. Electrically Tunale Excitonic Light-Emitting Diodes based on Monolayer WSe2 p-n Junctions. Nat. Nanotechnol. 2014, 9, 268–272. 40.

Varshni, Y. P. Temperature Dependence of the Energy Gap in Semiconductors. Physica

1967, 34, 149–154.


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Figure 1. Optical modulations in monolayer semiconductors. (a) The schematic diagram of optical transitions with the vertical electric field, FZ, and the profile of an energy shift, ∆EQCSE, derived from the quantum-confined Stark effect (QCSE). (b) The illustrations of the interplay between the reduction of the exciton binding energy and the bandgap renormalizations in the presence of carrier accumulations. The former causes a blueshift of optical transitions, ∆EB, and the latter yields a redshift, ∆EBGR, against an induced sheet carrier density, n2D.


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Figure 2. Electrical modulations of exciton PL in monolayer TMDCs. (a) The schematic illustrations of the PL measurement in an EDL capacitor with TMDC/graphite heterostructures, and the chemical structures of an ionic liquid and a tri-block copolymer. (b) The optical micrograph of CVD-grown monolayer WS2 on a graphite substrate. The red triangles represent the WS2 monolayer flakes. (c) The PL spectrum of monolayer WS2 on graphite (gray) recorded at room temperature and its fitted result by a single Lorentzian function (red) without voltage applications. (d) The applied voltage dependence of PL spectra for the monolayer WS2. (e) The 2D color map of the voltage dependence of the PL spectra.


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Figure 3. Exciton polarization and renormalization effects of monolayer TMDCs. (a-c) The variations of the PL peak energy shift (a), the PL emission intensity (b), and the PL spectrum broadening (c) as a function of the reference voltage, VR, measured in monolayer WS2. The results obtained from two different devices are shown. (d) The peak energy shifts (∆E) for monolayer WS2 as a function of the VR and the calculated FZ. The quadratic approximations are depicted by the pink line. It is noted that the ∆E is not following a quadratic manner at the higher VR regime shown by the gray symbols. The inset indicates the redshifts plotted against the square of FZ with their linear fitting. (e) The ∆E as a function of the calculated ∆n2D for both WS2 (green symbols) and MoS2 (purple symbols). The inset presents the comparisons between the obtained results and the reported values. The blue line exhibits the experimentally recorded photoinduced redshifts for WS2,19 and the gray line represents the theoretically predicted renormalization effects for MoS2 in the regime of ∆n2D > 1013 cm-2.20 The shaded red area is intended only to guide the eye for viewing the obtained results in this work.


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Figure 4. Electrical modulations of exciton and trion EL. (a) The optical micrograph (top) and the schematic illustration (bottom) of an electrolyte-based p-i-n light-emitting device with an exfoliated WSe2 flake. The monolayer (1L) WSe2 sample partially consists of the multilayer (ML) part. (b) The current-voltage (I-V) characteristic of a WSe2 light-emitting device at 280 K. The inset shows the logarithmic plot of the integrated EL intensity as a function of the injected current. The gray triangle displays the linear slope. (c) The comparison between the PL (gray) and EL (purple) spectrum measured at V = 2.4 V. (d) The voltage dependence of the EL spectra ranging from V of 2.4 V to 4.2 V. The inset presents the zoomed spectra of the main emission peaks. (e) The 2D color map of the voltage dependence of the EL spectra in the energy range from 1.5 to 1.75 eV. (f) The peak energy shifts for excitons (red, top) and trions (blue, bottom) as a function of the applied voltage. The blank symbols represent the results obtained from another sample.


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TOC


Exciton Polarization and Renormalization Effect for Optical Modulation

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