Article Cite This: J. Phys. Chem. C 2019, 123, 14097−14102
pubs.acs.org/JPCC
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
Downloaded via UNIV AUTONOMA DE COAHUILA on July 17, 2019 at 08:37:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The optical properties of monolayer materials can be indirectly tuned by the dielectric properties of surrounding materials. Although 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 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 Aexciton 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 band gap and exciton binding energies takes place but results only in a moderate shift of the exciton resonance energy because the influences of band gap and exciton energy shifts cancel each other to a large degree. Our results demonstrate the systematic tuning of optical properties by controlling the effective dielectric constant via the substrate, which is an important approach for the development of artificial materials.
1. INTRODUCTION Monolayer transition-metal dichalcogenides (TMDCs) are subject to intense research because of their ideal twodimensional (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 that 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 © 2019 American Chemical Society
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 band gap 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 microscopy image of the monolayer WS2 on the LiTaO3 substrate. The monolayer region is indicated by the broken line.
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 the realization of novel optoelectronic devices. Received: May 12, 2019 Revised: May 15, 2019 Published: May 15, 2019 14097
DOI: 10.1021/acs.jpcc.9b04514 J. Phys. Chem. C 2019, 123, 14097−14102
Article
The Journal of Physical Chemistry C
3. RESULTS AND DISCUSSION Figure 2a−e shows typical reflectance contrast (blue curve) and PL (red curve) spectra of the monolayers of WS2 on the five
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 red shift 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 the 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 band gap 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 provides a key to understand the physics behind the dielectric screening effects in atomically thin semiconductors and highlights the advantages of Coulomb engineering which enables tuning of the optical properties in monolayer TMDCs by designing unique vdW heterostructures.
Figure 2. (a−e) Typical reflectance contrast and PL spectra of monolayer WS2 on different substrates at 10 K. A curve fitting of each spectrum is performed using pseudo-Voigt functions for the peaks and a polynomial function for the background. The fitting result and each peak component are plotted in the same figure.
substrates at 10 K. The reflectance contrast is defined by ΔR/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 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
2. METHODS 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 the Supporting Information). An optical microscopy 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 band gap energy of the monolayer WS2. 14098
DOI: 10.1021/acs.jpcc.9b04514 J. Phys. Chem. C 2019, 123, 14097−14102
Article
The Journal of Physical Chemistry C
fabricated on the same kind of substrate, as shown in the 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 the SiO2/Si substrate, the extraction of the peak position by symmetric peak function might cause systematic error because the spectrum shape is somewhat changed because of the interference effect. Relative to the 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 (
