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Tunable Band-Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys Yanfeng Chen, Jinyang Xi, Dumitru O Dumcenco, Zheng Liu, Kazu Suenaga, Dong Wang, Zhigang Shuai, Ying-Sheng Huang, and Liming Xie ACS Nano, Just Accepted Manuscript • DOI: 10.1021/nn401420h • Publication Date (Web): 21 Apr 2013 Downloaded from http://pubs.acs.org on April 22, 2013
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Tunable Band-Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys Yanfeng Chen,1† Jinyang Xi,2† Dumitru O. Dumcenco,3† Zheng Liu,4 Kazu Suenaga,4 Dong Wang,2 Zhigang Shuai,2 Ying-Sheng Huang,3 and Liming Xie1* 1
Key Laboratory of Standardization and Measurement for Nanotechnology of Chinese Academy of
Sciences, National Center for Nanoscience and Technology, Beijing 100190, P. R. China 2
Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry,
Tsinghua University, Beijing 100084, P. R. China 3
Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei
106, Taiwan, Republic of China 4
Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba 305-8565, Japan †
These authors contributed equally.
* Address correspondence to:
[email protected] KEYWORDS: Two-dimensional material, alloy, transition-metal dichalcogenide, band gap, photoluminescence, density functional theory calculation
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ABSTRACT: Band gap engineering of atomically thin two-dimensional (2D) materials is the key to their applications in nanoelectronics, optoelectronics and photonics. Here, for the first time, we demonstrate that in 2D system, by alloying two materials with different band gaps (MoS2 and WS2), tunable band gap can be obtained in the 2D alloys (Mo1-xWxS2 monolayers, x=0-1). Atomic resolution scanning tunneling electron microscopy has revealed random arrangement of Mo and W atoms in the Mo1-xWxS2 monolayer alloys. Photoluminescence characterization has shown tunable band-gap emission continuously tuned from 1.82 eV (reached at x=0.20) to 1.99 eV (reached at x=1). Further, density functional theory (DFT) calculations have been carried out to understand the composition-dependent electronic structures of Mo1-xWxS2 monolayer alloys.
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Since the discovery of graphene in 2004,1 atomically thin two-dimensional (2D) materials have attracted broad interest in recent years because of their unique structures, versatile physical properties and potential applications.2-8 Towards applications in digital electronics and optoelectronics, sizable and tunable band gap is essential. So far, current 2D materials have limited choices of band gaps (0 eV for graphene, 5.8 eV for hexagonal BN monolayer,9 1.9 eV for MoS2 monolayer,10,11 2.1 eV for WS2 monolayer12). Many efforts, such as electrical gating,13 functionalization,14 strain15 and dielectric screening,16 have been made to tune the band gap of 2D materials. But still, very limited band gap range has been achieved. Alloying materials with different band gaps has been widely used in the band gap engineering of bulk semiconductors.17 Whether alloying can be used to engineer band gap of 2D materials is still unknown because no atomically thin 2D alloy was obtained. One work on 2D Ga-Si monolayer bound on Si(111) substrate showed only localized electronic states but no electronic mixing.18 However, theoretical calculations have shown that 2D alloys, such as hexagonal BNC monolayer19,20 and transition-metal dichalcogenide monolayer alloys,21 could have composition-dependent band gaps. Therefore, experimental work is needed to assess the electronic structure of 2D alloys. Experimental effort has been made on BNC alloys. However, BN and graphite are immiscible22 so that as-synthesized BNC films showed separated BN and graphene domains and no band gap tuning was observed.23 2D layered transition-metal dichalcogenide alloys may have better intermiscibility and were predicted to be stable.21 Several layered transition-metal dichalcogenide bulk alloys have been synthesized,24,25 such as Mo1-xWxS2 and Mo1-xWxSe2, suggesting good thermodynamic stability for the corresponding 2D alloys. Here, we have exfoliated the first family of atomically thin 2D alloys (Mo1-xWxS2 monolayers) and observed composition-dependent band gap photoluminescence (PL) from the monolayer alloys (1.82 to
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1.99 eV). A bowing26 was observed in the composition-dependent band gap emission of Mo1-xWxS2 monolayer alloys. Density functional theory (DFT) calculations have shown tunable band gap and band gap bowing for 2D Mo1-xWxS2 alloys, matching well with experimental results. RESULTS AND DISCUSSION Similar to MoS2 and WS227, Mo1-xWxS2 alloy has a layered structure28 and each layer consists of two hexagonal S atom layers and a sandwiched Mo/W atom layer. Mo or W atom sits in the center of a trigonal prismatic cage formed by six S atoms (Figure 1a). The atomic arrangements of Mo and W atoms in monolayer alloys were imaged by scanning transmission electron microscopy (STEM). Figure 1b shows a typical atomic-resolution STEM image of a Mo0.47W0.53S2 monolayer. The W content was determined by energy-dispersive X-ray spectroscopy (EDX) of the bulk crystals (Table S1). Due to its large Z number, W atom has a larger annual dark field (ADF) contrast and then is brighter than Mo atom,29 which is more clearly seen in the image after fast-Fourier-transform (FFT) filtering (Figure 1c). Electron energy-loss spectroscopy (EELS) characterization on individual atoms further confirmed that the brighter spots, showing two split O2,3 EELS peaks at 40-50 eV, were W atoms and the dimmer spots, showing one broad N2,3 EELS peak at 40-50 eV, were Mo atoms (Figure 1d, EELS of MoS2 and WS2 shown in Figure S1).30 By direct counting the numbers of Mo and W atoms in STEM images, the W content x can be directly calculated (0.53, totally 597 Mo and W atoms counted), well consistent with the W content value x of the bulk crystals (0.53, determined by EDX). The arrangements of Mo and W atoms in monolayer alloy are random (Figure 1b and 1c), which can be further quantified by a set of short-range order parameters αi,31 generally defined by 1
,
for AB binary alloys, where ,
is the probability that B atom’ ith nearest atom should be A atom (i is an integer), is the atom ratio of
A atoms. αi ranges from 1 to 1, while 0 value generally corresponds to random arrangement of A
and B. Analysis of STEM image (Figure S2) gives αi values of 0.01, -0.02, -0.03, 0.04 for i=1-4,
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respectively, revealing random arrangement of Mo and W atoms in the alloy. Systematic STEM imaging and disorder analysis for other Mo1-xWxS2 monolayer alloys (0
