Modification of Optical Properties in Monolayer WS2 on Dielectric

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C: Physical Processes in Nanomaterials and Nanostructures 2

Modification of Optical Properties in Monolayer WS on Dielectric Substrates by Coulomb Engineering Yuto Kajino, Kenichi Oto, and Yasuhiro Yamada

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04514 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 17, 2019

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The Journal of Physical Chemistry

Modification of Optical Properties in Monolayer WS2 on Dielectric Substrates by Coulomb Engineering Yuto Kajino, Kenichi Oto, and Yasuhiro Yamada* Department of Physics, Chiba University, Inage, Chiba 263-8522, Japan

Abstract: The optical properties of monolayer materials can be indirectly tuned by the dielectric properties of surrounding materials. While proof-of-concepts for this so-called Coulomb engineering have already been shown, the verification of the proposed effect for a wide range of different dielectric substrates is still missing. By employing reflectance and photoluminescence spectroscopy, we study the effect of the dielectric screening on the optical properties of a monolayer WS2 fabricated on five substrates with different dielectric constants. A systematic but moderate energy shift of the A-exciton resonance up to 40 meV is observed as the dielectric constant of the substrate increases. We consistently explain the shift in terms of the electron–hole Coulomb interaction due to dielectric screening. A theoretical calculation suggests that a significant modification of the bandgap and exciton binding energies takes place, but results only in a moderate shift of the exciton resonance energy because the influences of bandgap and exciton energy shifts cancel each other to a large degree. Our results demonstrate the systematic tuning of

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optical properties by controlling the effective dielectric constant via the substrate, which is an important approach for development of artificial materials.

1. INTRODUCTION Monolayer transition metal dichalcogenides (TMDCs) are subject to intense research due to their ideal two-dimensional (2D) nature and unique optoelectronic properties such as valley degree of freedom,1–3 extremely large exciton binding energy (~300 meV),4,5 and exciton many-body effects.6,7 Moreover, high performance of field effect transistors based on MoS2 has been reported.8 TMDCs attract growing attention as building blocks for van der Waals (vdW) heterostructures, that is, novel artificial 2D quantum structures possessing advanced functionalities and flexible designs.9 In contrast to conventional semiconductor quantum structures such as GaAs/AlGaAs quantum wells, vdW heterostructures are not restricted by the lattice matching condition owing to weak vdW force between the materials. This enables a highly flexible combination of different materials, corresponding to barriers in quantum well structures, can be chosen almost freely. In the course of recent research on vdW heterostructures, “Coulomb engineering” has been proposed as a new design concept, which is explained in the following.10–12 The Coulomb interaction between the photogenerated electron and hole that form an exciton is screened by the effective dielectric constant which is strongly affected by the dielectric constant of the surrounding material. The reason for this is that the electric force line generated by the electrons and holes reaches well beyond the upper and lower boundaries of the thin TMDC layer (see Figure 1a). Because the optical properties of TMDCs are dominated by excitons,4,13,14 it seems reasonable to tune the optical properties of atomically-thin materials by the surrounding material through Coulomb interaction. This idea offers a novel design principle for semiconductor heterostructures that will lead to realization of novel optoelectronic devices.


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The modification of optical properties in monolayer TMDCs by changing the surrounding dielectric constant using organic solvents and layered materials such as graphene, has already been reported.10,11,15 A common feature has been observed; the exciton resonance energies of the individual WS2, WSe2, and MoSe2 monolayers sandwiched with graphene or h-BN show a redshift compared with those observed when the monolayers are placed on SiO2.10,11 However, the detailed mechanism of the dielectric screening effect still remains unclear. This is partly because only a limited number of materials have been used for surrounding dielectrics. Further studies are needed to gain a quantitative and comprehensive understanding of Coulomb engineering concept. In this Letter, we study the change of the optical properties in monolayer WS2 by systematically changing the effective dielectric constant. By fabricating monolayers of WS2 on five kinds of substrates possessing different dielectric constants, we demonstrate the wide-range tuning of physical properties through Coulomb engineering. Depending on the substrate, a significant shift of exciton resonance energy (~40 meV) is observed. We estimate a large modification in the bandgap energy (~100 meV) based on the combination of optical measurements and theoretical calculation. The use of different substrates enables identification of the physical origin of the observed energy shifts. The investigated geometry, that is, a substrate on one side and vacuum on the other side of the monolayer TMDC, presents an interesting approach to Coulomb engineering. The observation of systematic changes in optical properties provide a key to understand the physics behind the dielectric screening effects in atomically-thin semiconductors, and highlight the advantages of Coulomb engineering which enables tuning of the optical properties in monolayer TMDCs by designing unique vdW heterostructures. 2. METHODS


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The monolayers of WS2 are fabricated by mechanical exfoliation from a bulk crystal on five types of substrates with different dielectric constants: SiO2(280 nm)/Si, Al2O3, LiTaO3, KTaO3, and SrTiO3. The last four substrates are transparent and colorless. Their static dielectric constants range from 4 to 300. The number of layers is then identified by optical microscopic observation, microscopic spectroscopy, and atomic force microscopy at room temperature (see Supporting Information). An optical microscope image of the monolayer WS2 on the LiTaO3 substrate is shown in Figure 1b. The monolayer WS2 is indicated by the broken line. To clarify the influence of the dielectric constants of the substrates on the monolayer TMDC, we perform microscopic reflectance and photoluminescence (PL) spectroscopy at 10 K, and compare the optical properties of the monolayers of WS2 on different substrates. A supercontinuum white light source (with a repetition rate of 80 MHz and a pulse duration of ~1 ps) generates the excitation light, which is focused to an approximately 1 µm spot by a coverslip-corrected objective lens (numerical aperture NA = 0.4). For reflectance spectroscopy, the white light is monochromatized by an acousto-optic tunable filter (bandwidth ~1 nm). For PL spectroscopy, the excitation photon energy is set to 2.32 eV, which is above the bandgap energy of the monolayer WS2. 3. RESULTS AND DISCUSSION Figures 2a–e show typical reflectance contrast (blue curve) and PL (red curve) spectra of the monolayers of WS2 on the five substrates at 10 K. The reflectance contrast is defined by ∆𝑅 𝑅 =

(𝑅𝑠𝑎𝑚𝑝𝑙𝑒 ― 𝑅𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒) 𝑅𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒, where 𝑅𝑠𝑎𝑚𝑝𝑙𝑒 and 𝑅𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 are the reflectances of the monolayer on the substrate and the bare substrate, respectively. It has been shown that the reflectance contrast spectrum of an atomically-thin film on a transparent substrate is approximately proportional to the absorption coefficient.16 In contrast to the transparent substrates, the reflectance


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contrast spectrum of the monolayer WS2 on the SiO2/Si substrate exhibits negative peaks on a broad background, which is due to an interference effect induced by the SiO2 layer.4 Figure 2a shows the results for the monolayer WS2 on the SiO2/Si substrate, which is the most common substrate for monolayer TMDC studies. The reflectance contrast spectrum exhibits strong negative peaks near 2.1 and 2.5 eV, which have been assigned to the so-called A and B excitons, respectively.4,17 The energy separation between the A and B excitons comes from the valence band splitting by spin-orbit interaction.18 The additional small peak observed below the resonance energy of the A exciton corresponds to the negative trion induced by unintentional residual doping.6,19,20 To accurately estimate the exciton resonance energies, we performed curve fitting using several pseudo-Voigt functions for the peaks indicated in each panel (three to five peaks as explained below) and a polynomial function for the background. The fitting results for the reflectance and PL data are shown with the blue and red solid curves, respectively. In the PL spectrum shown in Figure 2a, the A exciton and trion are observed, whose peak energies are identical to those observed in the reflectance contrast spectrum. Well below the A-exciton energy we observe a strong and broad PL peak, which is probably due to defect states.21,22 Just below the trion energy, the data also reveals two additional peaks, which we label P1 and P2 in the order from high to low energies. Although similar peaks have been reported so far, the origins of these peaks are still under discussion.21–25 Because the P1 and P2 peaks appear significantly under strong excitation, we speculate that they reflect intrinsic electronic states such as those of the biexciton and trion fine structure induced by conduction band splitting.23,25 Note that the A-exciton emission intensity from the monolayer WS2 is relatively weak compared to the low-energy peaks. The reason for this weak emission is the quenching at low temperatures by the ground state dark exciton located a few meV below the bright exciton state.26,27


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The results of the curve fitting for the reflectance contrast and PL spectra of the monolayer WS2 on the other substrates are shown in Figure 2b–e. From this data set we can confirm that the Aexciton peak energy is independent of the type of measurement in all samples. We note that, in the data for the KTaO3 and SrTiO3 substrates, the peak intensities of trion, P1, and P2 are not strong enough to be resolved well. Therefore, some of the peaks are not identified and others have large errors of the fitting parameters. We plot the averaged values and deviations of the peak energies extracted from the reflectance contrast and PL spectra in Figure 3a. Note that we observed small sample-to-sample differences in the peak energies even when the monolayers are fabricated on the same kind of substrate, as shown in Supporting Information. The dielectric constant of the substrate (x-axis) increases from left to right. The A-exciton resonance energy monotonically increases with the substrate dielectric constant with the exception of SiO2/Si. Note that, in the case of SiO2/Si substrate, the extraction of the peak position by symmetric peak function might cause systematic error, because the spectrum shape is somewhat changed due to interference effect. Relative to peak energy observed for the SiO2/Si and Al2O3 substrates, a blue shift of approximately 40 meV is observed for the substrate with the largest dielectric constant. This energy shift is much larger than the sample-tosample differences on the same kind of substrate (< 20 meV). We can also exclude the possibility of an exciton–trion crossover, which has been previously reported in the PL shift of MoS2 surrounded by different organic solvents,15 since the observed energy shift is much larger than the trion binding energy of 20-30 meV. To verify the influence of the substrate-dependent sample quality, we evaluated the average A-exciton full width at half-maximum (FWHM) as shown in Figure 3b. (Statistical data are shown in Supporting Information.) There is no correlation between FWHM and peak energy, suggesting that the sample quality has almost no influence on the peak


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shift. Furthermore, the systematic A-exciton peak shift in Figure 3a cannot be explained by lattice distortion due to the mismatch between the thermal expansion coefficients of the monolayer WS2 and substrate (see Supporting Information). In addition, we consider that the impact of carrier doping on the peak energy shift is negligible. Carrier doping level can be roughly estimated from trion/exciton intensity ratio for each substrate. As shown in Figure 2, there is almost no correlation between doping level and peak shift. This suggests that doping makes almost no impact on the Aexciton peak shift observed in our study. It is interesting to note that the resonance energy shifts of the trion, P1, and P2 states shown in Figure 3a are similar to the those observed for the A exciton. Also for these states, the resonance energies estimated from PL and reflectance measurements agree well with each other. We consider that the mechanism responsible for the energy shifts of trions and biexcitons is the same as that for the A exciton. Since the resonance energies of excitons and trions systematically shift with the substrate dielectric constant, it is suggested that these changes in the optical properties result from the dielectric screening effect. By using these results, we demonstrate that it is possible to consistently describe the tuning of optical properties in monolayer WS2 by the dielectric screening effect. However, we cannot discuss the physical origin of this energy shift solely based on the substrate-dependent exciton resonance energy, because the dielectric screening effect should modify both the exciton binding energy and the bandgap energy. Thus, it is essential to evaluate the influence of the dielectric screening on the bandgap and exciton binding energies as proposed in previous theoretical works.28–31 We perform a theoretical calculation of the exciton binding energy by numerically solving the 2D hydrogen model with a screened Coulomb potential between electrons and holes as described by the RytovaKeldysh potential28,29


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[(

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)]

(1 + 𝜀𝑠)𝑟 (1 + 𝜀𝑠)𝑟 𝜋𝑒2 𝑉𝑒ℎ(𝑟) = ― 𝐻0 ― 𝑌0 ,where 𝐻 and 𝑌 are Struve and Bessel 2𝑟0 2𝑟0 2𝑟0 0 0

) (

functions, and 𝑟0 is the screening length which is related to the 2D polarizability of the monolayer WS2. This potential results in the conventional 1 𝑟 dependence for a large separation between electron and hole (r ≫ 𝑟0) but a weaker log (𝑟) dependence for a small separation (r ≪ 𝑟0). Here, we use 𝑟0 = 7 nm based on a previous study.32 𝜀𝑠 is the dielectric constant of the substrate. Because the exciton binding energy is much larger than the phonon energy of the substrate, we can neglect the phonon contribution to the dielectric screening. Thus, it is more appropriate to take the dielectric constant at near infrared (IR) frequencies rather than at far IR frequencies.33 In this research, we employ the substrate’s dielectric constant at near IR frequencies (a few hundred THz) and vary 𝜀𝑠 in a wide range (SiO2: 1.9, Al2O3: 2.8, LiTaO3: 4.2, KTaO3: 4.6, SrTiO3: 4.8).34–39 Based on density functional theory, we assume that the A-exciton reduced mass is 0.16 𝑚0 where 𝑚0 is the electron rest mass.40 Figure 4 shows the calculated exciton binding energy and experimentally obtained A-exciton resonance energy as a function of the effective dielectric constant, which is defined by 𝜀𝑒𝑓𝑓 =

(1 + 𝜀𝑠) 2. The calculation predicts a reduction of the A-exciton binding energy as the effective dielectric constant increases. From the exciton binding energy and the A-exciton resonance energy, we can predict the bandgap energy (see Figure 4). We confirm a large red shift of the bandgap energy on the order of 100 meV. This red shift, which is known as bandgap renormalization, is consistent with the theoretical prediction by GW and Hartree-Fock calculations.41,42 Our results suggest that the dielectric screening effect largely modifies both the exciton resonance energy and the bandgap energy. The experimentally observed exciton resonance energy, however, changes less because the changes in the bandgap and exciton binding energy have opposite influences on


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the resonance energy.43,44 Since in reality various influences besides the dielectric screening are possible, the verification of the dielectric screening model for different substrates is needed to identify the physical origin of the observed energy shifts. Finally, we consider the limitations of tuning optical properties by Coulomb engineering. Due to the large exciton binding energy of the monolayer WS2, the dominant factor for dielectric screening is the dielectric constant of the substrates at near IR frequencies. The dielectric constant above the LO phonon energy, which usually corresponds to the far IR region, is relatively small (typically on the order of 1 to 10) compared with the static dielectric constant, because it has no contribution of phonons and is mainly determined by electronic polarization. Thus, the dielectric screening effect is quite limited as long as the exciton binding energy is larger than the LO phonon energy of the substrate. However, if the exciton binding energy is smaller than the LO phonon energy of the substrate, the dielectric screening grows abruptly by phonon contribution and this should result in a huge impact on the optical properties. For this purpose, realization of larger effective dielectric constants is required. 4. CONCLUSION In conclusion, we demonstrated the tuning of optical properties in monolayer WS2 through the dielectric screening effect for five substrates with different dielectric constants. The observed changes in the exciton resonance energy result from both the shift of the bandgap and the exciton binding energy, and we estimated a significant tuning of the bandgap energy by combining the experiment with calculation results. This bandgap energy shift, which is induced by surrounding dielectric constants (e.g. vacuum on top and the different substrates below), is essential for the fabrication of appropriate vdW heterostructures and development of future nanoscale optoelectronic devices.


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Figure 1. (a) Illustration of the exciton in the monolayer WS2 on substrates with different dielectric constants, and the corresponding energy diagrams. Both the exciton binding energy Eb and the bandgap Eg decrease with an increase in the dielectric constant of the substrate. Therefore, the change of the exciton resonance energy Ex is small. (b) Optical microscope image of the monolayer WS2 on the LiTaO3 substrate. The monolayer region is indicated by the broken line.


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Figure 2. (a–e) Typical reflectance contrast and photoluminescence spectra of monolayer WS2 on different substrates at 10 K. A curve fitting of each spectrum is performed using pseudo-Voigt


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functions for the peaks and a polynomial function for the background. The fitting result and each peak component are plotted in the same figure.

Figure 3. Substrate dependence of (a) peak energies and (b) the A-exciton FWHM extracted from the reflectance and PL spectra. Peak energies and FWHM are statistically averaged over the data obtained for the same substrate with the exception of LiTaO3 and KTaO3. The broken lines are eye guides. Diamonds and circles correspond to data obtained from reflectance and PL, respectively.


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Figure 4. The A-exciton resonance energy Ex and exciton binding energy Eb obtained from experiment and theoretical calculation, respectively. Shift of the bandgap energy Eg estimated from combination of the exciton binding energy Eb and the A-exciton resonance energy Ex with respect to the effective dielectric constant. The broken lines are eye guides. ASSOCIATED CONTENT Supporting Information Identifying number of layers (Figure S1); Origin of P1 and P2 peaks in PL spectrum (Figure S2); Impact of thermal expansion on optical spectra (Table S1); Statistics of peak energies and the A exciton FWHM (Figure S3)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions All authors contributed to the preparation of the manuscript approved the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to thank Prof. Takashi Nakayama for the helpful discussions and Masaya Arai for help with the sample fabrication. Part of this work was supported by JSPS KAKENHI Grant Number 15K17678. REFERENCES (1)

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Modification of Optical Properties in Monolayer WS2 on Dielectric

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