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Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2

Qicheng Zhang,† Carl H. Naylor,‡ Zhaoli Gao,‡ Ruizhe Wu,† Irfan Haider Abidi,† Meng-Qiang Zhao,‡ Yao Ding,† Aldrine Abenoja Cagang,† Minghao Zhuang,† Xuewu Ou,† and Zhengtang Luo*,†

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†

Department of Chemical and Biomolecular Engineering, the University of Hong Kong Science and Technology, Clear Water Bay, Kowloon, Hong Kong ‡ Department of Physics and Astronomy, University of Pennsylvania, 209S 33rd Street, Philadelphia, Pennsylvania 19104 6396, United States S Supporting Information *

ABSTRACT: The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important. KEYWORDS: molybdenum disulfide, photoluminescence, trion, 2D materials, doping

M

Among the TMDC family, molybdenum disulfide (MoS2) is the most widely studied, as its large crystals are found in natural minerals,16 and more importantly, large monolayer MoS2 is produced by chemical vapor deposition (CVD).17 To use MoS2 for sensing applications, the main method is to control their electrical or optical response with doping, including electrochemical doping or doping with the target molecules.18−20 However, for trion emission, only intensity is found to be tunable at room temperature, but photon energy is also tunable at low temperature.7 Some studies at room temperature thus regard photon energy of trions differs to excitons by a constant binding energy term during the electrical tuning,21 while lowtemperature study reports that this energy difference strongly increases with MoS2 Fermi energy due to the recoil effect.7 In this paper, we discovered two trion-like photoemission peaks at room temperature. However, photon energy of one is sensitive to electrical tuning, while the other does not. These two peaks originate from two recoil processes upon trion recombination, where the states accepting recoiled electrons are

onolayer semiconducting transition-metal dichalcogenides (TMDCs) are a family of semiconductors with direct bandgaps,1−3 in which the strong lightmatter interaction and optical properties make them as promising materials for optoelectronics.4−6 The low dielectric constant and the two-dimensional geometry of TMDC monolayers lead to a poorly screened Coulomb potential, which leads to the formation of excitons and trions due to the strong many-body effects.7−10 On the other hand, the direct band gap at the K point has valley difference and a large spin splitting at the valence band due to the breaking of inversion symmetry and spin−orbital coupling.2,11−14 Nevertheless, significant differences exist between excitons and trions. For instance, the excitons are reported to emit coherently while the exchange-splitting in trions destroys such coherence.10 Moreover, Raman signal is enhanced when the emission energy is in resonance with neutral exciton emission, but such a phenomenon has not been observed in trions.15 To understand unusual optical properties of TMDCs and further development of optoelectronic devices, it is imperative to clarify the physics and methods that can be used to control the interactions between electrons, excitons, phonons, and defects. © 2017 American Chemical Society

Received: May 17, 2017 Accepted: November 9, 2017 Published: November 9, 2017 10808

DOI: 10.1021/acsnano.7b03457 ACS Nano 2017, 11, 10808−10815

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Figure 1. Fabrication of electrochemically gated van der Waals heterostructure. (A) Electrochemical cell is fabricated by enclosing it with silicon substrate, transparent ITO glass, and PDMS walls. Graphene film extends outside of the PDMS walls and contacts the metal electrodes. A saturated Ag/AgCl reference electrode is inserted through the PDMS wall. (B) Raman spectra for monolayer graphene MoS2 and their fabricated heterostructure. After stacking, characteristic Raman peaks for both MoS2 and graphene remain. (C) Frequency mapping of E12g and A1g peak showing their peak position are apart by less than 20 cm−1, evidencing that MoS2 triangle is monolayer, with occasionally another layer present at the center. (D) MoS2 monolayer measured by atomic force microscope (AFM), showing a thickness around 0.7 nm (upper inset). The lower inset displays a typical optical image for the MoS2 before transfer. The color scale goes from 0 (dark) to 3 nm (bright).

excite the heterostructure through the ITO window. Lastly, an electrolyte solution was injected into the predefined channel, which guided into an electrolyte reservoir installed with a reference electrode. The electrolyte solution connects to the reference electrode situated at the reservoir and flow between the MoS2/graphene and ITO glass. Before the fabrication of devices, a thorough characterization was performed to ensure the quality of the 2D materials. The layer number of graphene was determined by its Raman spectra, along with atomic force microscopy (AFM) measurements. The measured full width half-maximum (fwhm) of its 2D peak (at ∼2700 cm−1) is ∼30 cm−1, indicating the monolayer nature.25 This is corroborated by that fact that its 2D peak intensity is much higher than that of its G peak (∼1580 cm−1) (Figure 1B) with 514 nm excitation wavelength, typical for monolayer graphene.25 Similarly, the layer number of MoS2 was confirmed by its Raman spectroscopy and AFM. The majority of the flakes are found to be monolayer, showing the 1 signature 18 cm−1 in difference between the E2g and A1g 26,27 modes, as shown in Figure 1B, except in the center part of the MoS2 triangle, where the E12g and A1g modes are more than 20 cm−1 apart (Figure 1C), indicating the presence of multilayers. The layer number was further confirmed by AFM measurement, showing a height of ∼0.7 nm per layer (Figure 1D). After graphene and MoS2 are stacked, E12g and A1g modes of MoS2 become further apart, and the fwhm of the 2D peak of graphene is increased to around 40 cm−1 (Figure 1B) due to strain and interlayer interactions.26,28 Figure 2A illustrates the resultant PL spectra at different doping level of the MoS2/graphene stack with 632.8 nm laser

a bound state and the Fermi level, respectively. The presence of bound states leads to an additional final state for recoiled electrons from trions, thus splitting photoemission for trions. This discovery indicates the importance of bound states in MoS2 due to the recoil effect of trion recombination, where the capability of electrical tuning and temperature dependent behavior are significantly influenced.

RESULTS AND DISCUSSION In order to study the photoluminescence (PL) of monolayer MoS 2 , we first fabricated the FET devices by using heterostructures of graphene and MoS2. To facilitate a high electron-doping level, we resorted to ion gate in liquid phase, where the high capacitance of ion electric double layer makes it possible to achieve high carrier density at relatively low gate voltage. The graphene layer is used as a gate electrode to provide a simple gating. Figure 1A illustrates the configuration of the MoS2/graphene heterostructure device. Briefly, this device is a three-electrode electrochemical cell that resembles a scanning electrochemical microscope,22 in which the MoS2/ graphene heterostructure acts as the working electrode, directly facing the counter electrode, i.e., the ITO glass. Here, graphene and MoS2 (and hBN) were all grown by chemical vapor deposition (CVD) using an established procedure routinely performed in our laboratory.23,24 Second, MoS2 and graphene were layer-by-layer transferred by using the standard PMMAassisted wet transfer method to a 1 cm × 1 cm silicon wafer with 300 nm silicon oxide layer.24 Third, a patterned PDMS channel was placed on the silicon wafer, followed by stacking with a piece of transparent ITO glass, which allows the laser to 10809

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Figure 2. Photoluminescence (PL) of MoS2/graphene stack. (A) The room temperature PL spectra of graphene/MoS2 heterostructure as a function of gate voltage (shown in legend). The positions of two strongest peaks are marked by dash lines at 1.85 and 1.82 eV respectively. (B) PL spectra fit with three Lorentzian peaks, with the summed peak in purple, in contrast to the peak at ∼1.85 eV that is fixed in position, while the peak at ∼1.82 eV slightly red-shifts with increasing gate voltage. A new peak appears when the intensities of the other two weaken. (C) Schematic drawing quasiparticles of MoS2, exciton, and trion. (D) Intensity mapping of PL peaks. The PL spectra are split into three peaks according to their photon energy and displayed individually. The peaks are denoted as I, II, and III, corresponding to the peaks annotated in B. The color scales of II and III are the same, while intensities in I have been reduced to 1/3 to give better contrast.

Figure 2C) and trion A− (formed by two electrons and a hole shown in Figure 2C) are present in this range of photon energy. To better illustrate the variation of photon energies and emission intensities of peaks I−III, we plot the intensity map against VG of the three separated peaks shown in Figure 2D (combined mapping in Figure S2). The separation of peaks II and III is evident by combining Figure 2B and the different trends upon doping. We observe that the highest intensities of peaks I and II are located at VG of 0.20 and 0.35 V, respectively. A significant difference is found for peak III, whose intensities remain almost unchanged across a wide range of VG from 0.3 to 0.8 V. If we only consider the range of VG < 0.36 V, the PL change is exactly the same as that of bare MoS2 previously reported at room temperature,18,19,29 with only peaks I and II assigned to the exciton and trion emissions, respectively. The photon energy of peak I does not change, and that of peak II slightly changes within 10 meV. However, the red-shift of peak III is apparent with an apparent inclination, more similar to that of trion emission reported at 10 K.7 To make sure whether the PL peaks come from the heterostructures or from MoS2 itself, we changed the sequence of the stack to graphene/MoS2 and inserted a layer of hBN in between. An apparent phenomenon is that photon energy difference between peaks II and III (i.e., ΔE23) against VG is dependent on their configurations, as indicated in Figure 3A. This confirms that these three peaks are not from electrochemical reactions, which depends only on chemical potential, thus the gate voltage. When switching the stacking order from

excitation. The electron doping level is tuned by adjusting positive gate voltage (VG) (also refer to the Supporting Information section 5 and Figure S8 for MoS2 Raman change). At low doping levels, those PL spectra of the heterostructure display similarities to that of bare MoS2,7 and a previously reported MoS2/graphene heterostructure.21 A readily observable feature in the spectrum is a very large peak appearing between 1.76 and 1.92 eV, centered at 1.85 eV (hereafter referred to as peak I). Another smaller peak centered ∼1.82 eV (peak II) becomes prominent when VG is in the range of 0.26 and 0.31 V. When VG equals 0.36 V or higher, a third peak (peak III) appears at much lower photon energy, concurrent with the gradual disappearing of peaks II and I. To further elucidate the nature of the bound states, we measured the linear polarization dependent PL using two linear polarizers at the excitation and detection light path, respectively. The linearly polarized PL measurement result is summarized in Figure S9B. This figure shows a linearly polarized peak, and nonpolarized two humps, consistent with the hypothesis of the existence of three peaks (Supporting Information section 5). On the basis of this observation, the PL spectra are deconvoluted accordingly as shown in Figure 2B. As can be seen from the deconvoluted peaks, in contrast to peaks I and II, where the peaks are almost fixed, peak III significantly shifted, and the variation is strongly dependent on the gate voltage. These features are different from that described in the literature7,21 where only two peaks from exciton A (formed by an electron and a hole shown in 10810

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Figure 3. PL of MoS2 in different configurations of heterostructures. (A) Plots of gate voltage against photon energy difference between peak II and peak III for different configurations. Slopes and intercepts are different from each other. (B) Evolutions of PL spectra from different configurations (MoS2/graphene, black; graphene/MoS2, red; graphene/hBN/MoS2, blue) are similar. Specifically, at the gate voltage which gives an exact position for peak II, the positions of peak III are the same for all configurations, as is the intensity ratio between peaks II and III. Peaks II and III are indicated by the dash-dot lines. (C) Plot of energy difference of peaks II and III against their intensity ratio. All peaks follow the same trend, as indicated by the green line. (D) Plot energy differences of peaks I and II against intensity ratios for various MoS2 stacking configurations. Similar trends are obtained as indicated by the green line.

thickness of dielectric interlayer. As shown in Figure 3C, a plot of ΔE23 as a function of intensity ratio follow the same trend, regardless of stacking sequence and thickness of dielectric interlayer. Peaks I and II have the similar correlation as shown in Figure 3D. The change of peak intensity ratio between peaks I and II varies with the energy difference between these two peaks ΔE12 in a monotonous way within a fitting error of around 0.4 meV. Slight deviation may be a result of screening.29,30 These findings strongly suggest that peaks I, II and III are intrinsic peaks to MoS2, because the PL spectra change is independent of heterostructure configurations, taking into account the actual sensitivity of Fermi energy against gate voltage. Furthermore, contributions of interlayer exciton31−34 could be excluded, where electron−hole pair at different layers of graphene/MoS2 heterojunction should have energy smaller than 1.2 eV,35 far smaller than the present range. It is worth noting that the spectra evolution is not changed even when a layer of h-BN is sandwiched between graphene and MoS2, while the interlayer exciton between graphene and MoS2 should be greatly weakened with the increased distance between the two active layers.36 Also, the intensity trends are different for all three peaks, as shown in Figure 2D, thus rules out the possibility that the peaks are phonon replicas.37

MoS2/graphene to graphene/MoS2, the slope increases from 0.0187 V/meV to 0.0455 V/meV, making ΔE23 less sensitive to gate voltage. An additional layer of hBN in-between further decreases the sensitivity slightly, resulted in a slope of 0.0447 V/meV (or no change, since 95% confidence bound is around ±0.003 V/meV), and a slight decrease in intercept. These behaviors could be understood from the view of Fermi energy of MoS2, where the stacking order changes electrostatic environment for MoS2. Insertion of an hBN layer changes the capacitance between graphene and MoS2, and slightly modify the sensitivity for different configurations (Supporting Information section 2.3, Figure S4). All the changes induced by configuration variation could be thus explained by the consequent change of relationship between MoS2 Fermi level and the gate voltage, but not due to the possible interaction between MoS2 and graphene layers. Therefore, the emission peaks are intrinsic to MoS2 and primarily vary with its Fermi energy. Moreover, the evolutions of emission spectra are quite similar for all the configuration. Humps of the three peaks described previously are all visible for each configuration, indicating none of the three peaks are exclusive to MoS2/ graphene configuration. Figure 3B demonstrates such a set of spectra where humps for peaks II and III are visible and all PL are at the same position regardless of stacking sequence and 10811

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Figure 4. Doping level dependence of peaks for MoS2/graphene heterostructure. (A) Photon energies of peaks II and III vary almost linearly against the estimated MoS2 Fermi level. Slopes for peaka II and III are 0.14 and 0.45, respectively. (B) Peak intensities vary with doping level. Peak I is only apparent at low doping level, while peaks II and III appear with a moderate doping. Furthermore, peak II disappears with high doping level, while the intensity of peak III changes slightly. The lines are trends for the points. (C) Schematic drawing of trion recombination process and the effects of final states. In the left image, one electron combined with the hole in a trion and the other electron recoils. A shallow defect state is presented which traps the recoiled electron from trion recombination. In the right image, due to the Pauli exclusion principle, the recoiled electron should goes up to the Fermi level. (D) Graph demonstrates the doping dependence of estimated energy levels of final states of recoiled electrons after trion recombination. The upper branch reflects the energy level of recoiled electrons from “free” trions, e.g., Fermi level. The lower branch reflects the “effective energy level” of bound states which accept recoiled electrons from “bound” trions. Bound states are energetically preferred to states at Fermi level, resulting in dimmer upper branch at low doping level. When occupancies of the bound states are increased, or when the screening of defects becomes more significant, the possibilities of recoiled electrons from trions to bound states decrease, resulting in the darkening of lower branch. The linear relationship of Fermi energy to electron density is indicated by the dashed line in the upper branch. The color map is adapted from the photoluminescence, with scaled intensity, as an indication of state occupancy by recoiled electron.

graphene and MoS2 does not appear in our PL spectra as insertion of dielectric hBN layers has no apparent effect, the presence of graphene will modify the dielectric environment for MoS2 and red-shift the excitons emission (compare Figures S8A and S10A) because the photon energy of excitons and emissions is sensitive to the dielectric environment.29 Based on the above findings, we assign all three peaks to excitons and trions. The exciton and trion peaks could be judged according to their photon energy and intensity change behavior against doping.7,10 Excitons are neutral, and they will react with additional electrons to give trions. The intensity of trions could be related to that of excitons by mass action law. The mass action law can be calculated as21,29

By far, we found three peaks by doping level dependent PL. These features are independent of the heterostructure configurations. However, the use of graphene as contact has significant advantages as opposed to conventional metal contact, showing relatively weaker quenching and mild van der Waals interactions. Generally, graphene gives a uniform carrier distribution for efficient gating and does not severely quench the PL. On the contrary, conventional source−drain metal contacts are not suitable for efficient doping in our liquid gate system (Figure S10A), where MoS2−electrode contact breaks possibly due to small bubbles causing a tear or detachment. If graphene is substituted with gold, the PL quenching is severe (Figure S10B), but the doping level dependent PL is quite similar to what we found for MoS2/ graphene (Figure S10C), with all asymmetric peaks found. However, for gold contact, there’s an additional doping insensitive asymmetric peak, possibly due to multimodal plamsonic effects.38 Although short-range interaction between

⎛ E A− ⎞ NAne 4mA mekBT = exp ⎜− ⎟ NA− π ℏ 2m A− ⎝ kBT ⎠ 10812

(1) DOI: 10.1021/acsnano.7b03457 ACS Nano 2017, 11, 10808−10815

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ACS Nano where NA and NA− represent the number of exciton A and trion A−; ne is the number of electrons; mA, mA−, and me represent the mass of exciton A, trion A−, and electron, respectively; EA− is the binding energy for trion; and kB, T, and ℏ represent Boltzmann’s constant, temperature, and reduced Planck’s constant, respectively. Provided that PL intensities of exciton A and trion A− have IA− = ΓA−NA− and IA = ΓANA. ΓA− and ΓA are the emission rate for trion and exciton and are constants. Therefore IA I A−

⎛ E −⎞ 1 ∝ exp⎜ − A ⎟ ne ⎝ kBT ⎠

smaller than expected but close to trion binding energy (18 meV7). Moreover, the energy of peak II is higher than that of trions (peak III) and thus not likely to be a bound state of trions because defect binding lowers the energy of trions. As peak II possesses part of the properties of trions as it appears with moderate electron doping, an assignment of peak II requires taking into account of the recoil effect of trions. The photon energy of trions are related to that of excitons by7,44 ωA − ω A− = E A− + E F

where ωA and ωA are the photon energy of exciton and trion, respectively, and EA− and EF are the trion binding energy and Fermi energy of MoS2, respectively. The part of EF contributes to linear red-shift of trion A−7 and could be understood as energy required to recoil the additional electron up to Fermi level where other states are occupied. It should be noted that the recoil effect itself contributes to the photon energy change, while following steps do not participate in the emission process.7 If we neglect the many-body interaction canceled by making energy subtraction between excitons and trions, we could write the photon energy of trion as

(2)

The intensities of excitons are strong in neutral MoS2 but quickly decrease upon increase of electron density. Peak I fits this criteria quite well (Figures 2D and 4B) and is assigned to excitons.1 At high doping level, trion emission is reported to appear and be stable, and its photon energy decreases directly as the Fermi energy changes.7 To clarify the peak emission from trion, Fermi energy dependence of emission spectra has been studied by a MoS2/graphene configuration for its wide tuning window and sensitivity of ΔE23. We characterized the relationship between gate voltage and MoS2 Fermi energy by Raman G-peak of graphene,35,39,40 which is reported to shift linearly with graphene Fermi energy.41 On MoS2-covered graphene, charge transfer makes the G-peak shift much slower than bare graphene (Figure S6A). The density of carriers in MoS2 can be extracted by ⎛ nG + n MoS nG ⎞ 2 ⎟ + E F(nG) e(VG − VNP) = e 2⎜⎜ + C TG C MoS2 ⎟⎠ ⎝

(4)

−

ω A− = Eg − EA − E A− − E F

(5)

where Eg is the band gap and EA is the binding energy of exciton. The band structure of MoS2 changes slightly against perpendicular field (Supporting Information section 4). However, the final state of the released additional electron is a special factor affecting the phonon energy. Figure 4C illustrates two final states of recoiled electrons. The electrons could either go to states of Fermi level or bound states. If a bound state for electron exists slightly above the conduction band minimum, the photon energy of trions ω′A− of which additional electrons are recoiled to this bound state could be written as

(3)

where e is the electron charge, VG is the gate voltage, VNP is the voltage reaching neutral point of graphene, nG and nMoS2 are the carrier density in graphene and MoS2, respectively, CTG and CMoS2 are the capacitance for top gate (electrolyte) and monolayer MoS2, respectively, and EF(nG) is the Fermi energy of graphene. CTG is calculated from cyclic voltammetry of pure graphene, and nG can be calculated from the G-peak shift calibrated by bare graphene G-peak shift (Supporting Information section 3). The mapping of gate voltages to MoS2 carrier densities for MoS2/graphene heterostructures is explicitly given in Figure S6D. The doping level from 0.36 to 0.56 V in Figure 2B is calculated to be in the range of ∼0.7 × 1013 to ∼1.2 × 1013 /cm2 for peaks II and III to be prominent. Figure 4A plots the energy shift of peaks II and III as a function of MoS2 Fermi energy. The linear fit gives a slope value of 0.14 and 0.45 for peaks II and III, respectively. The latter is very close to the slope of the reported low temperature trion A− at high Fermi energy.7 Moreover, the intensity of peak III also remains stable across a wide window at high Fermi energy (Figure 4B). These two features are typical for trions.7 Peak II deviates from normal trion emission by its weaker dependence on Fermi energy and by its highly fluctuating intensity at high doping level. As shown in Figure 4b, the intensity of peak II undergoes a slight increase followed by a slow decrease. Peak II is also not likely to be originated from intrinsic defects that create defect bound states of excitons which are reported to be at ∼0.2 eV lower than that of exciton42 comparable with exciton binding energy estimated at ∼0.6 eV.14,43 We observe that the energy difference between peak II and that of excitons (peak I) is ∼30 meV, an order

ω A′− = Eg − EA − E A− − E D + E DB = Eg − EA − E A− − E F′ (6)

EBD

where ED is the energy level for the bound state, is the binding energy, and EF′ = ED − EBD could be viewed as “effective energy level” compared to EF in eq 5. The energy of such a bound state is higher than the “free” trions by ω′A− − ωA− = EBD − ED + EF and smaller than free excitons by ωA − ω′A− = ED + EA− − EBD. Therefore, the energy difference ωA′′− − ωA− increases rapidly with Fermi energy EF. The other photon energy difference ωA − ω′A− increases slightly with EF because EBD may slightly decrease due to increased electron screening at high doping level. On the other hand, the “effective energy level” E′F is smaller than Fermi energy EF, making ω′A− energetically favored against ωA−. But at high doping level, such states are either filled by free electrons or screened. All these derived features fit well with the observed peaks I, II, and III, assigned as ωA, ω′A− and ωA−, as shown in Figure 4D. Assuming the energy of exciton follows the same trend as demonstrated previously by Mak et al.,7 the estimated EA + EA− + EF and EA + EA− + EF′ could be derived from the corresponding differences of photon energies in eqs 4−6). These two terms reflect the energy level of final states of recoiled electrons after trion recombination, with two additional slightly varying binding energy terms. An almost linear dependence of term EA + EA− + EF against doping level is provided by the EF term (also refer to eq S11). The energyfavorable bound states for recoiled electrons become either screened or filled at high doping level, leading to decreasing 10813

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Other Measurements. Optical measurements are done by a Leica DMLM microscope. The AFM was measured under the tapping mode of a nanoscope-multimode/dimension scanning probe microscope.

possibility to host recoiled electrons, making states at the Fermi level more favorable. More interestingly, ω′A− could be hardly observed at low temperature because the bound states are preferred to be occupied by free electrons, while thermal activation at room temperature makes them partly available to recoiled electrons from trions. Such a “bound” trions are different from the bound excitons (usually bound by defects). If we use ωA′ to denote a shallow bound exciton, then ωA − ω′A = EAD, where EAD is the binding energy of defect state and decreases with EF due to the screening at high doping level. But that of ωA − ω′A− and thus ΔE12 increase with doping level. Therefore, the doping level dependence of peak II is different from what a defect bound exciton is expected. Defects which could bind excitons are also able to bind recoiled electrons to form such “bound” trions. The presence of such states may change some properties of MoS2, say, photoconductivity, and requires further confirmation and study.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03457. Detailed device fabrication method, discussion of the electrostatics of the heterostructures, calculation of charger carriers, and supporting data for the control experiment (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Qicheng Zhang: 0000-0002-3414-4083 Zhengtang Luo: 0000-0002-4822-9694 Notes

CONCLUSIONS In summary, we report a discovery of two trion-like photoemission peaks intrinsic to MoS2 from electrochemically gated MoS2 in a van der Waals heterostruture at room temperature. By studying peak evolution against MoS2 Fermi energy, we propose that the two trion-like peaks result from different trion recombination processes, where the recoiled electrons are either captured by a bound state or going up to Fermi-level. The former is less electrically tunable, but dominant at moderate doping level. The latter only appears at high doping level but is more sensitive to electrical tuning. This study reveals previously neglected complexity of TMDC photoemission at room temperature due to the recoil effect of trion recombination.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project is supported by the Research Grant Council of Hong Kong SAR (Project No. 16204815). We appreciate support from Center for 1D/2D Quantum Materials and the I n n o v a t i o n a n d T ec h n o lo g y Co m m i ss i o n ( I T C CNERC14SC01 and ITS/267/15). Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated. REFERENCES (1) Splendiani, A.; Sun, L.; Zhang, Y. B.; Li, T. S.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271−1275. (2) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (3) 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. U. S. A. 2005, 102, 10451−10453. (4) Sundaram, R. S.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A. C.; Avouris, P.; Steiner, M. Electroluminescence in Single Layer MoS2. Nano Lett. 2013, 13, 1416−1421. (5) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (6) Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Castro Neto, A. H.; Novoselov, K. S. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311−1314. (7) Mak, K. F.; He, K. L.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2012, 12, 207−211. (8) Ross, J. S.; Wu, S. F.; Yu, H. Y.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. D. Electrical Control of Neutral and Charged Excitons in a Monolayer Semiconductor. Nat. Commun. 2013, 4, 1474. (9) Wang, G.; Bouet, L.; Lagarde, D.; Vidal, M.; Balocchi, A.; Amand, T.; Marie, X.; Urbaszek, B. Valley Dynamics Probed Through Charged and Neutral Exciton Emission in Monolayer WSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 075413. (10) Jones, A. M.; Yu, H. Y.; Ghimire, N. J.; Wu, S. F.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu,

METHODS Fabrication of the Electrochemically Gated System. MoS2 and graphene were layer-by-layer transferred by using the standard PMMA-assisted wet transfer method to a 1 cm × 1 cm silicon wafer with 300 nm silicon oxide layer. A patterned PDMS channel was then placed on the silicon wafer, followed by stacking with a piece of transparent ITO glass. Solution of 0.5 M NaCl was injected into the predefined channel, going through tube into an electrolyte reservoir installed with a reference electrode. A more detailed procedure is described in the Supporting Information. Measurement of the Raman Spectra and PL Spectra. Raman spectra were captured by a Renishaw InVia system where a grating of 1800 l/mm was used. The excitation wavelength was 514 nm. PL spectra were captured by a Princeton Instrument Acton SP2750 monochromator equipped with PIX 100BR CCD. A grating of 600 l/ mm was used for most PL spectra, which produces highest sensitivity to the interested photon energy range, while a 150 l/mm grating was used for larger photon range. The excitation wavelength used here was 632.8 nm. The in situ PL measurement together with gate voltage was made by an integration of Princeton Instrument Acton SP2750 and Keithley 2614B source meter. Laser passed through an Olympus microscope equipped with long working-distance objective lens, shining on the heterostructure. One channel of the source meter was used to apply and measure voltage and current in the three electrode configuration, and another channel was used to supply syncing signal to the monochromator and controlling computer. The three electrode configuration is realized by the four-probe mode of Keithley 2614B. The reference electrode is connected to the sense terminals. 10814

DOI: 10.1021/acsnano.7b03457 ACS Nano 2017, 11, 10808−10815

Article

ACS Nano

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DOI: 10.1021/acsnano.7b03457 ACS Nano 2017, 11, 10808−10815