MoS2-MoO2-MoO3 Core-Shell Belt

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Functional Nanostructured Materials (including low-D carbon)

Novel Red Emission from MoO3/MoS2-MoO2-MoO3 Core-Shell Belt Surface Lei Wang, Xiaohong Ji, Ting Wang, and Qinyuan Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13784 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Applied Materials & Interfaces

Novel Red Emission from MoO3/MoS2-MoO2-MoO3 Core-Shell Belt Surface

Lei Wang†, Xiaohong Ji*†, Ting Wang†‡, and Qinyuan Zhang*†‡ †

School of Materials Science and Engineering, South China University of Technology,

Guangzhou 510641, China ‡

State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication

Materials, South China University of Technology, Guangzhou 510641, China

ABSTRACT: Electrically driven red emission from MoS2-MoO2-MoO3 (MS-MO) hybrid-based metal-semiconductor-metal (MSM) devices is first reported. MoO3 belts with high crystal quality and sufficient size are synthesized by thermal deposition. A layer of MS-MO hybrid is then produced on the belt surface to form MoO3/MS-MO core-shell by sulfurization. The devices exhibit unique electrical properties, nonlinear I-V curve and electric hysteresis characteristics at high applied biases (>2.4 V), where MS-MO hybrids act as electron transport channels. The electroluminescent current of the device increases to a set current limit over time when a constant bias is applied. The novel characteristics of the device are attributed to the space charge limited conduction (SCLC) mechanism occurring in MS-MO hybrids. The strong light emission is from recombination of excitons within the MoS2 phase. This work develops a simple and effective method to drive MoS2 to emit light on a large scale without using monolayer MoS2 and vertical p-n junctions, indicating great potential for future 2D optoelectronics and photonics applications.

KEY

WORDS:

MoS2,

MoS2-MoO2-MoO3

hybrids,

Electroluminescent

semiconductor-metal structured device, Space charge limited conduction

1 Environment ACS Paragon Plus

emission,

Metal-

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Molybdenum disulfide (MoS2), a typical layered transition metal dichalcogenide (TMD) semiconductor,1 has attracted research attention due to its excellent electrical and optical properties and potential range of applications complementary to those of graphene, such as use in catalysis,2 transistors,3 and photovoltaics.4 The layers of bulk TMD crystals are held together by weak van der Waals forces; thus, obtaining single- to few-layer crystal flakes is possible by a mechanical exfoliation method. Bulk MoS2 is a semiconductor with an indirect band gap of 1.3 eV, and monolayer MoS2 has a direct band gap of 1.8 eV.5,6 The synergistic effects of novel optical and electrical properties7,8 make monolayer MoS2 a promising candidate for light-emitting devices. For example, Sundaram et al.9 reported electrically excited luminescence in monolayer MoS2, and the electroluminescence (EL) was due to a hot carrier process. Ponomarev et al.10 explored ambipolar light-emitting transistors on monolayer MoS2, and the EL emission was due to exciton recombination in MoS2. However, the EL emissions in both cases were limited to the region of the contacts. Alternatively, vertical p-n junctionbased light emitting diodes (LED) were developed to overcome the insufficient effective area of the contacts. For instance, LEDs based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon have been reported and show light emission from the edge11 and entire surface of the heterojunction.12 To date, only a few studies have focused on LEDs, which is possibly due to the difficulties in fabricating controllable and scalable monolayer MoS2. Recently, hybrids of molybdenum compounds have attracted intense interest because of their unique properties and potential in optoelectronics and clean energy applications. For example, the reciprocal hybridization of MoO2 and few-layer MoS213 and nanocarved MoO2 and MoS2 hybrids14 were fabricated as anode materials for lithium-ion batteries (LIBs) due to the synergistic effect of both MoO2 and MoS2. Hybrids of MoS2 and MoO2 that act as active catalysts in the hydrogen evolution reaction (HER) have also been reported.15-17 In addition to hydrogenation and LIB applications, MoS2based material hybrids also show great potential for LEDs. For instance, an n-type SiC/p-type MoS2MoO3 heterojunction was developed as an active layer for an LED.18 In this work, MoO3/MoO2-MoS2-MoO3 (MS-MO) core-shell belts were developed by vulcanizing a single-crystalline MoO3 belt in a S atmosphere. The morphology, composition, and optical and electrical properties of the core-shell belts were investigated. A unique, electrically excited


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luminescence from the surface of the MoS2-MoO2-MoO3 hybrids is realized. The underlying emission mechanism is discussed in detail.

Results and discussion MoO3 belts were synthesized by thermal deposition, and the MoO3 belts tended to attach to the wall of the tube. Then, the MoO3 belts were transferred onto a Si substrate for further sulfurization at 480 °C for 20 min. The illustration of the setup is shown in Figure 1a. The inset in Figure 1b is the digital photo of an as-grown MoO3 belt. The MoO3 belt is transparent and white and differs from the pale-yellow MoO3 powder. The size of the belt is approximately 20 mm×5 mm, and the thickness of the belt is estimated to be tens of microns. Figure 1b shows the XRD patterns of the MoO3 belts before and after sulfurization. The strong diffraction peaks at 13º, 23.4º, 26º, 39.2º can be indexed to the (020), (110), (040), and (060) planes of orthorhombic MoO3 (JCPDS, card no: 5-0508), respectively, and the lattice constants a, b, and c are 3.96, 13.86, and 3.70 Å, respectively. The sharp diffraction peaks of (020), (040), and (060), which belong to {010}, indicate the excellent single-crystalline nature of the belt. The additional peaks at ~33.1º and ~37º were detected after sulfurization of the belt, and these peaks match the 2H-MoS2 (100) (JCPDS card no: 37-1492)14 and monoclinic MoO2 (200) planes (JCPDS card no: 65-5758),13 respectively. The XRD results revealed the formation of MoS2 and MoO2 phases on the MoO3 belts after sulfurization. Figure 1c compares the Raman spectra of the MoO3 belt before and after sulfurization. Ten Raman peaks at ~113, 156.7, 196, 215, 282, 335, 363, 669, 818, and 996 cm-1 were observed for the as-grown MoO3 single crystal (marked as the symbol #), and these peaks are in good agreement with the Raman vibration modes of α-MoO3.19,20 However, six other Raman peaks were observed for the Sannealed MoO3 belt. The peaks at ~130, 495, 572, and 737 cm-1 (marked as ∆) correspond to monoclinic MoO2,21 and the Raman peaks at ~383.7 and 407 cm-1 (marked as Ψ) belong to 2H-MoS2.7 For 2H-MoS2, the E12g peak at ~383.7 cm-1 corresponds to the in-plane vibration mode of molybdenum and sulfur atoms, and the A1g peak at 407 cm-1 corresponds to the out-of-plane vibration mode of the sulfur atoms, as shown in the inset. The frequency difference between A1g and E12g is ~23.3 cm-1,



indicating that the MoS2 on the surface is multilayer.22 Thus, the existence of MoS2 and MoO2 on the surface of the belt is further confirmed by the Raman results.

Figure 1. (a) Illustration of the setup for preparing the as-grown MoO3 belts. (b) XRD patterns of the MoO3 belts before and after sulfurization. The inset in Figure b is the digital photo of an as-grown MoO3 belt. (c) Raman spectra of the MoO3 belt before and after sulfurization. Figure 2a is a typical plain-view SEM image of the MoO3 belt and shows a smooth surface and clear boundary contour. To obtain information about the microstructure, the MoO3 belt was ground into small grains for the TEM analysis. Figure 2b is a high resolution TEM (HRTEM) image of the inset belt, showing a complete grain in the image. The lattice spacings of ~0.36 nm and ~0.39 nm correspond to the MoO3 (001) and (100) planes. Figure 2c shows the selected area electron diffraction (SAED) pattern of the belt. The regular arrangement of the SAED pattern confirms the excellent single-crystalline feature of the MoO3 belt. Figure 2d and 2e are SEM images of MoO3 after sulfurization. A novel, crater-like surface can be observed from the tilted-view SEM image in Figure 2d and shows the obvious difference in the surface morphology in comparison with that of the MoO3 belt. In particular, a grating-type stripe structure can be observed in the magnified plain-view SEM image in Figure 2e. To further investigate the atomic crystal structure of the belt surface, a thin layer of the sulfurized belt was used for the TEM characterization. Figure 2f presents a HRTEM image of the sample. The lattice spacing of 0.62 nm corresponds to the 2H-MoS2 (002) planes; The lattice spacings of ~0.36 nm and ~0.39 nm correspond to the MoO3 (001) and (100) planes. the lattice spacing of 0.24 nm is consistent with the monoclinic MoO2 (200) planes; and the lattice spacings of ~0.36 nm and ~0.39 nm correspond to the MoO3 (001) and (100) planes. Most of the surface area is occupied by


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MoS2 and MoO2 grains, which are a few nanometers in size. The corresponding SAED pattern of the annealed belt is shown in Figure 2g. In addition to the diffraction from the MoO3 single crystal, two concentric rings were present and assigned to MoS2 (100) and MoO2 (002), which proved that the sulfurized belt was composed of monocrystalline MoO3, polycrystalline MoS2 and MoO2. The TEM analysis is consistent with the XRD and Raman results.

Figure 2. (a) SEM image, (b) HRTEM, and (c) SAED pattern of the as-grown MoO3 belt. (d)-(e), Tilted-view and plain-view SEM images of the S-heated MoO3 belt. (f) HRTEM image of the Sheated MoO3 belt; The insets are magnified HRTEM images, showing the lattice spacings of MoO2 and MoO3 phases. (g) SAED pattern of the S-heated MoO3 belt. XPS characterizations were performed on a sulfurized MoO3 belt. All the XPS data were calibrated using the carbon C 1s peak (284.6 eV) as a reference. The XPS full-scan spectrum of the belt is presented in Figure 3a with the compositions of C, Mo, S, and O elements. Figure 3b and Figure 3c display the Mo-3d and S-2p core-level spectra, respectively. The curves were well-fitted by Gaussian deconvolution. The peaks of Mo-3d5/2 at a binding energy of ~232.4 eV and Mo-3d3/2 at ~235.6 eV are characteristic of molybdenum in a 6+ oxidation state, which is from the Mo-O bond of α-MoO3.23 The characteristic peaks centered at ~229.2 and ~232.7 eV were assigned to the doublet



Mo-3d5/2 and Mo-3d3/2, respectively, of 2H-MoS2, while the binding energy peaks at ~231.2 and 234.2 eV can be attributed to the doublet Mo-3d5/2 and Mo-3d3/2, respectively, of MoO2.24 Furthermore, the S-2p3/2 peaks at ~161.9 eV and S-2p1/2 peak at ~162.9 eV confirm the existence of 2H-MoS2 on the surface of the belt,25 as shown in Figure 3c. The peak at ~168 eV corresponds to the S-O bond, which can be attributed to the absorbed SO2 on the surface. Thus, the surface of the sulfurized MoO3 belt is mainly composed of MoO2 and 2H-MoS2. The Raman mapping analysis of the cross-section of the belt further demonstrated this composition, as shown in the supporting information in Figure S1. The Raman shifts at 390-410 cm-1 and 810-825 cm-1 are characteristic of MoS2 and MoO3, respectively. The Raman mapping intensity reveals the core-shell structure of the belt and the MoO3 core. The presence of MoO2 and MoS2 phases on the surface is attributed to the sulfurization process. The chemical reaction should be 2MoO3+4S=MoS2+MoO2+2SO2↑. A contrast experiment annealing the MoO3 microbelt at 480 °C under an Ar atmosphere was performed. The Raman spectrum of the Ar-annealed MoO3 belt only exhibits an α-MoO3-related Raman signal, as shown in supporting information Figure S2, which proves that the sulfurization reaction occurred during the S annealing process. Based on the above structural and compositional analyses, the sulfurized MoO3 belt is a MoO3/MoS2-MoO2-MoO3 hybrid core-shell belt.

Figure 3. (a) XPS full-scan spectrum, (b) Mo-3d and (c) S-2p of S-annealed MoO3 belt.


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Figure 4. (a) The structure illustration of the device. (b) The typical I-V curve of the belt device with silver paste contacts; the inset is the photograph of the device. (c) The I-T curve of the belt device; the inset V-T curve corresponding to the marked region of the I-T curve. (d) The corresponding lightemission process at a bias of 7 V with a limiting current of 700 mA. An EL device based on the MoO3/MS-MO belt was constructed with silver paste electrodes fixed at two ends of the core-shell belt. The schematic diagram of the device is shown in Figure 4a. The electrical properties of the MoO3/MS-MO belt were tested at room temperature. Figure 4b is the typical current−voltage (I−V) characteristics of the device as shown in the inset photograph in Figure 4b. The entire I-V curve exhibits obvious nonlinear behavior. To explore the underlying physical mechanism, the I-V curve was divided into two regions, α (voltages in the range of -3 to +~5.5 V) and β (voltages larger than ~5.5 V). The linearity of the I-V curve in region α indicates ohmic contact behavior and reveals an ohmic conduction mechanism with a small bias voltage between the electrodes and the belt.26,27 With the increasing bias voltage, the I-V curve of the device shows peculiar



nonlinear behavior when the voltage is greater than ~5.5 V (the region β in Figure 4b), which follows the relation I∝V2. The typical I-V and I-V2 curves of the device are shown in the supporting information in Figure S3. The I-V performance of the as-grown MoO3 belt was measured for comparison. The resistance of the as-grown MoO3 belt is ~5000 MΩ, which is much larger than that of the MoO3/MS-MO belt (~ 30 Ω), as shown in the supporting information in Figure S4. The excellent conductivity of the MoO3/MS-MO belt is beneficial to the surface layer of the MoS2-MoO2-MoO3 hybrid, which acts as the main transmission channel. If a bias is applied to a low-mobility semiconductor and the injected charge density exceeds the intrinsic free carrier density of the material, the space charge limited conduction effect (SCLC) will occur.28,29 According to the Hall effect measurements, the MoO3/MS-MO belt is an n-type conductor with a carrier concentration of ~1021 cm3

and mobility of

MoS2-MoO2-MoO3 Core-Shell Belt

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