Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe2/WSe2 van der Waals Heterostructures Pramoda K. Nayak,†,‡ Yevhen Horbatenko,§ Seongjoon Ahn,†,‡ Gwangwoo Kim,†,‡ Jae-Ung Lee,∥ Kyung Yeol Ma,†,‡ A-Rang Jang,†,‡ Hyunseob Lim,⊥ Dogyeong Kim,# Sunmin Ryu,# Hyeonsik Cheong,∥ Noejung Park,*,§,∇ and Hyeon Suk Shin*,†,‡,§ †
Department of Energy Engineering and Department of Chemistry, ‡Low Dimensional Carbon Materials Center, and ∇Department of Physics, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea § Center for Multidimensional Carbon Materials (CMCM), Institute of Basic Science (IBS), Ulsan 44919, Republic of Korea ∥ Department of Physics, Sogang University, Seoul 04107, Republic of Korea ⊥ Department of Chemistry, Chonnam National University, Gwangju 61186, Republic of Korea # Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea S Supporting Information *
ABSTRACT: Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe2/ WSe2 vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe2 and WSe2 layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe2 and WSe2. KEYWORDS: transition-metal dichalcogenide, van der Waals heterostructure, photoluminescence spectroscopy, twist angle, interlayer exciton, density functional theory
T
vdW HSs, one needs to consider the lattice mismatch between the components as well as the band gap and energy level of each component, as these can influence the characteristics of the vdW HSs. vdW HSs consisting only of TMDs are promising for use in nanoelectronics and optoelectronics.16 The relatively large exciton binding energy of TMDs (0.1−1 eV)17,18 has a significant effect on their optical properties, resulting in strong light−matter interactions.19 It has been predicted that a number of TMD vdW HSs exhibit the so-called type-II band alignment, in which the
wo-dimensional (2D) van der Waals (vdW) heterostructures (HSs), which are formed by stacking different types of 2D layered materials through the weak interlayer vdW interaction, have recently attracted significant interest since they allow for unique functionalities previously not possible in homogeneous bulk materials.1 With respect to the various 2D materials available, such as graphene, transition-metal dichalcogenides (TMDs), and hexagonal boron nitride (h-BN), diverse combinations have been suggested for vdW HSs. For example, graphene/h-BN,2,3 MoS2/graphene,4−8 MoS2/h-BN,9,10 WS2/h-BN,11,12 and graphene/h-BN/graphene13−15 vdW HSs have been demonstrated, and their diverse physical properties, which originate from the interlayer interaction, have been discussed. When designing © 2017 American Chemical Society
Received: January 28, 2017 Accepted: March 31, 2017 Published: March 31, 2017 4041
DOI: 10.1021/acsnano.7b00640 ACS Nano 2017, 11, 4041−4050
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a Schematic front view (a) and side view (b) of MoSe2/WSe2 HSs with different twist angles (0°, 13°, 28°, 47°, and 60°). The Mo and W atoms in the horizontal positions are represented by the gray and purple spheres, respectively. The Se atoms in the vertical position are represented by the green spheres. The angle between the two lines connecting the centers to W and Mo atoms is considered as the twist angle.
explained by the characters of the wave functions at VBM, as determined by density functional theory (DFT) calculations. The variation of interlayer coupling between the layers also resonates with low-frequency (LF) Raman analysis. The highsymmetry stacking configuration corresponding to twist angles close to 0° and 60° exhibits the shear mode (SM), while a mismatched atomic alignment with intermediate angles results in the layer breathing mode (LBM). To further highlight the importance of the extent of interlayer coupling, we incorporated a monolayer of h-BN between the MoSe2 and WSe2 layers and found that the PL of the interlayer excitons was quenched completely even for angles of 0° and 60°, owing to the increased separation, which weakened the interlayer interaction.
conduction band minimum (CBM) and the valence band maximum (VBM) are located in different layers.20−23 As a result, photoexcited electrons and holes can be localized in the band edges in the different layers after rapid relaxation.20 Such bound electron−hole pairs observed in TMD vdW HSs are known as interlayer excitons and are different from the intralayer excitons observed in individual TMD monolayers or homobilayers. It is possible to optically pump these interlayer excitons through PL measurements performed on a well-coupled heterostructure system and engineer the excitons to allow for efficient optoelectronics.16,23−27 The features of interlayer excitons have been studied in a few vdW HSs, including in MoSe2/WSe2,23 MoS2/WS2,24,25,27,28 MoS2/ WSe2,16,26 and WSe2/WS229 vdW HSs. However, while the aforementioned interlayer excitons have been observed in a few TMDs vdW HSs with only coherent twist angles (0° or 60°)28 or those with arbitrary random stacking,16,23,24,26−29 a systematic study of the effect of the twist angle on the interlayer excitons is lacking. In the case of TMD homobilayers, the optical responses (absorption and PL spectra) and their dependence on the twist angle, mainly attributable to the intralayer excitons, have been studied intensively.30−32 Several previous studies combining PL measurements with ab initio calculations investigated the MoS2 bilayer and found that the transition across the indirect band gap depends more on the twist angle than that across the direct gap.32 This increase in understanding regarding TMD homobilayers has naturally led to curiosity about TMD vdW HSs with a high degree of lattice matching, particularly regarding the effects of the twist angle. As stated in the previous section, the presence of interlayer excitons in TMD vdW HSs has been observed by a few research groups;16,23−28 however, their evolution under different degrees of interlayer rotation and how they are affected when a dielectric layer is incorporated at the interface remain unknown. In this paper, we report the evolution of interlayer excitons in MoSe2/WSe2 vdW HSs for various degrees of interlayer rotation (twist angles). We show that the interlayer excitons are highly susceptible to the strength of the interlayer interaction between the two constituent layers. The PL of the interlayer excitons is enhanced for twist angles close to 0° and 60° but disappears at intermediate twist angles. This can be
RESULTS AND DISCUSSION Triangle-shaped MoSe2 and WSe2 monolayers were grown on c-plane sapphire substrates by chemical vapor deposition (CVD).33−36 In brief, MoSe2 (or WSe2) monolayers were grown by the vaporization of MoO3 (or WO3) and Se powders in a 2 in. quartz tube furnace in a controlled gaseous environment (see the Methods section for details). A schematic of the growth process and the characteristics of the MoSe2 and WSe2 monolayers are shown in Figures S1 and S2, respectively. The crystallinity of CVD grown monolayer samples has been confirmed from the uniform PL intensity within a single flake (Figure S3). The MoSe2/WSe2 vdW HSs were prepared by transferring such crystalline triangle-shaped monolayers onto SiO2 (300 nm)/Si substrates via a two-step process. In the first step, the as-grown WSe2 monolayer was detached from the sapphire substrate and transferred onto a SiO2/Si substrate by the conventional transfer technique using poly methyl methacrylate (PMMA). After the PMMA had been removed, the MoSe2 monolayer grown on sapphire was transferred onto the WSe2/SiO2/Si substrate using the same transfer technique. Finally, a MoSe2/WSe2 vdW HS was formed on the SiO2/Si substrate by removing the PMMA. The conventional PMMAassisted transfer technique is shown in Figure S4. In the asprepared MoSe2/WSe2 vdW HSs, the degree of coupling between the two layers was not high, possibly owing to the presence of the adsorbates and residuals formed at the interface during the transfer process.27 Since the majority of the adsorbates and residuals can be removed by thermal annealing 4042
DOI: 10.1021/acsnano.7b00640 ACS Nano 2017, 11, 4041−4050
Article
ACS Nano
Figure 1. Raman signatures of the MoSe2/WSe2 HSs. (a) LF Raman spectra of HSs with different twist angles (0° ≤ θ ≤ 60°) and those of the individual TMD layers at room temperature, measured using a 532 nm laser. The positions of the shear mode (SM) and layer-breathing mode (LBM) are shown by the vertical dashed lines. An unknown peak is observed near the SM (marked with asterisk) in the WSe2 monolayer, which is not assigned yet (see the text and Figure S8). (b) Measured Raman shifts in the SM and LBM with the twist angle. (c) Raman peak intensities of the SM and LBM versus the twist angle. Note that the Raman intensities were normalized with respect to that of the Si peak at 520.6 cm−1. HF Raman spectra of the HSs for different twist angles for frequencies of 230−270 cm−1 (d) and 280−360 cm−1(e) along with those of the individual monolayers. The positions of the Raman active modes A1′, E′, 2LA (M) and the inactive modes A2″ along with that of the Si peak are marked with vertical dashed lines.
at elevated temperatures,26 the as-prepared MoSe2/WSe2 vdW HSs were annealed in vacuum (
