Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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WO3−WS2 Vertical Bilayer Heterostructures with High Photoluminescence Quantum Yield Biyuan Zheng,†,‡,⊥ Weihao Zheng,‡,⊥ Ying Jiang,‡,⊥ Shula Chen,† Dong Li,† Chao Ma,† Xiaoxia Wang,‡ Wei Huang,‡ Xuehong Zhang,‡ Huawei Liu,‡ Feng Jiang,‡ Lihui Li,‡ Xiujuan Zhuang,‡ Xiao Wang,‡ and Anlian Pan*,†,‡
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†
Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, People’s Republic of China ‡ School of Physics and Electronics, Hunan University, Changsha, Hunan 410082, People’s Republic of China S Supporting Information *
heterostructures is proved to be an efficient way to engineer the physical properties, while maintaining the intrinsic nature of each component. Therefore, developing heterostructures with specific structures would be an efficient way to realize devices with high PLQY. In this work, ultrahigh PLQY was demonstrated in WO3− WS2 bilayer heterostructures, which were achieved through an improved PVD process. The as-grown heterostructures showed uniform coverage of WO3 monolayers on the surface of largescale WS2 monolayers, with the PL emission of WS2 being greatly enhanced by a factor of 116-fold as compared to the pristine WS2 monolayers prepared by the same method. The PLQY of the heterostructures can reach a record high of ∼11.6%. Time-resolved PL (TRPL) experiments demonstrated the PLQY improvement of WS2 originates from the transition of the trions to excitons along with the suppression of the nonradiative recombination. The approach of employing onestep growth WO3−WS2 heterostructures with ultrahigh PLQY may find important applications in future high-efficiency lightemitting devices. For the growth of the WO3−WS2 heterobilayers, the mixture of WS2 and WO3 powders were placed at the heating zone of a high temperature tube furnace as the source (Figure 1a). The SiO2/Si was placed at the downstream as the substrate. The growth temperature was kept at 1100−1150 °C for 10 min for the growth of the heterostructures. Crystalline structures and optical images of the WO3−WS2 heterostructures are shown in Figure 1b and Figure 1c,d, respectively. The lateral size of the achieved heterostructures can reach up to 600 μm. Morphology of the partially covered heterostructure can be clearly identified by the larger magnification scanning electron microscopy (SEM) image (inset in Figure 1c), which indicates that the WO3 monolayers with circular/oval shapes are uniformly distributed on the surface of WS2. Importantly, the completely covered bilayer WO3−WS2 heterostructures can also be synthesized (Figure 1d). WS2 monolayers have been proven to be the better templates to obtain thin layer materials than SiO2/Si.7,11 Atomic force microscopy (AFM) was employed to identify the thickness of the WO 3 −WS 2
ABSTRACT: Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive for applications in a wide range of optoelectronic devices, due to their tremendous interesting physical properties. However, the photoluminescence quantum yield (PLQY) of TMDCs has been found to be too low, due to abundant defects and strong many-body effect. Here, we present a direct physical vapor growth of WO3−WS2 bilayer heterostructures, with WO3 monolayer domains attached on the surface of large-size WS2 monolayers. Optical characterizations revealed that the PLQY of the as-grown WO3−WS2 heterostructures can reach up to 11.6%, which is 2 orders of magnitude higher than that of WS2 monolayers by the physical vapor deposition growth method (PVD-WS2) and about 13-times higher than that of mechanical exfoliated WS2 (ME-WS2) monolayers, representing the highest PLQY reported for direct growth TMDCs materials so far. The PL enhancement mechanism has been well investigated by time-resolved optical measurements. The fabrication of WO3−WS2 heterostructures with ultrahigh PLQY provides an efficient approach for the development of highly efficient 2D integrated photonic applications.
T
ransition metal dichalcogenides (TMDCs), have attracted considerable interests for future optoelectronic applications due to their novel physical properties. From bulk to monolayers, TMDCs show an indirect−direct band gap transition, leading to the excitonic light emission and therefore making them well suitable in various optoelectronic devices.1−15 However, because of the abundant defects and strong many-body effect,16−18 the photoluminescence quantum yield (PLQY) of pristine TMDCs monolayers is extremely low compared to traditional semiconductor materials, which restricts its further applications. Therefore, how to improve the PLQY of TMDCs monolayers becomes an important issue that should be addressed. Previous studies have indicated that chemical treatment, oxygen bond doping and electrostatic doping can improve the PLQY of TMDCs monolayers.18−26 However, these methods are complicated. Construction of © XXXX American Chemical Society
Received: March 30, 2019 Published: July 12, 2019 A
DOI: 10.1021/jacs.9b03453 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 1. (a,b) Schematic of the synthesis process and threedimensional schematic representation of the WO3−WS2 heterostructure. (c,d) Typical optical image of (c) partially covered and (d) completely covered WO3−WS2 heterostructures. The inset in panel c shows the SEM image of the area within red box. (e−g) AFM images of the (e,f) partially covered and (g) completely covered heterostructure. The insets give the corresponding height profile.
Figure 2. (a) TEM-EDX profile of the WO3−WS2 heterostructure. (b−d) XPS spectra of W 5p, W 4f peaks and S 2p peaks of the WO3− WS2 and pristine WS2, respectively. (e,f) HRTEM images of the WS2 monolayer and a WO3 nucleus, respectively. Scale bar: 2 nm. The insets in panels e and f show the corresponding FFT-diffraction patterns.
heterostructure, and Figure 1e−g depicts the morphology of top WO3 in different scales, indicating that the thickness of the top WO3 and the WO3−WS2 were about 0.8 and 2.1 nm, respectively. The AFM image in Figure S1 demonstrates that the bottom WS2 is a monolayer. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) characterizations confirmed the element information of the WO3−WS2 heterostructures. The TEM energy-dispersive X-ray spectroscopy (EDS) collected from WS2 (blue line in Figure 2a) corresponds to the characteristics of the WS2.8,10 However, the spectrum (black line in Figure 2a) taken from the heterostructure region shows higher elements intensity in W and O signals, which confirmed the formation of WO3−WS2 heterostructures.27,28 XPS characterizations indicate the chemical states of the WO3−WS2 heterostructures. Figure 2b−d shows the XPS survey spectra (Figure 2b), the binding energy of the W (Figure 2c) and S (Figure 2d) of the heterostructure and the pristine WS2. As shown in the lower half part of Figure 2c, the peaks located at 33.46, 35.63 and 38.67 eV were ascribed to W4+ 4f7/2, W 4+ 4f5/2 and W 4+ 5p5/2 lines of the WS 2 nanosheets.27−29 While for the heterostructures, addition peaks at 36.45 and 38.02 eV were observed, corresponding to the W6+ 4f7/2 and W6+ 4f5/2, respectively, agrees with WO3.28,29 The element information for S is shown in Figure 2d. There are two peaks located at 162.69 and 163.9 eV, which can be attributed to S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions, respectively (Figure 2d). Figure 2e,f gives the high resolution TEM (HRTEM) images and the corresponding fast Fourier transform (FFT) results collected from the WS2 monolayer and the WO3 nucleus, respectively, confirmed the atomic feature of the heterostructures.27,28
Microphotoluminescence spectroscope was used to investigate the optical properties of the WO3−WS2 heterostructures, with the PVD-WS2 and the ME-WS2 monolayers as the references. Their optical images are provided in Figure 3a−c. Notably, different from the WS2 monolayer, the central area of WS2 covered with WO3 monolayers can be clearly identified (Figure 3a). The PL intensity mappings are given in Figure 3d−f. As shown in Figure 3d, the central WO 3 −WS 2 heterostructure regions exhibit much brighter PL emission in comparison with the uncovered peripheral region, which indicates the PL enhancement effect induced by the WO3. PL mappings of the PVD-WS2 and the ME-WS2 monolayers were also examined for comparison, which show much weaker PL intensity in similar conditions (Figure 3e,f). The dark-field PL images in the insets of Figure 3d−f, respectively, show the redlight emission in the bilayer WO3−WS2 heterostructures, the PVD-WS2, and the ME-WS2 monolayers, in which the strongest PL emission was observed in heterostructures. The slightly brighter edge emission found in WO3−WS2 heterostructure and PVD-WS2 monolayer can be attributed to the oxygen atom absorption.30 Such PL enhancement observed in the heterostructure region was also corroborated in the completely covered WO3−WS2 heterostructures (Figure S2). Typical PL spectra under an excitation power density of 1.25 W·cm−2 collected from the three samples are displayed in Figure 3g for quantitative comparison. The PL spectrum of the PVD-WS2 monolayer shows an obviously lower PL intensity with the red-shifted peak position (∼13.9 meV) as compared B
DOI: 10.1021/jacs.9b03453 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
shows an obvious higher PLQY and varies a little with modulating the excitation power. In the range of
