Defect Engineering in Single-Layer MoS2 Using Heavy Ion Irradiation

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Defect Engineering in Single-Layer MoS2 Using Heavy Ion Irradiation Zuyun He, Ran Zhao, Xiaofei Chen, Huijun Chen, Yunmin Zhu, Huimin Su, Shengxi Huang, Jianming Xue, Junfeng Dai, Shuang Cheng, Meilin Liu, Xinwei Wang, and Yan Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17145 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 17, 2018

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ACS Applied Materials & Interfaces

Defect Engineering in Single-Layer MoS2 Using Heavy Ion Irradiation Zuyun He1, Ran Zhao2, Xiaofei Chen3, Huijun Chen1, Yunmin Zhu1, Huimin Su4, 5, Shengxi Huang6, Jianming Xue7, Junfeng Dai4,5, Shuang Cheng1, Meilin Liu8, Xinwei Wang2 * and Yan Chen1 * 1Guangzhou

Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research

Institute, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China 2School

of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen

518055, China 3

China Institute of Nuclear Information & Economics, Beijing 100871, China

4 Department

of Physics, Southern University of Science and Technology, Shenzhen 518055,

China 5

Shenzhen Institute for Quantum Science and Engineering, Southern University of Science and

Technology, Shenzhen, 518055, China 6Department

of Electrical Engineering, The Pennsylvania State University, University Park,

Pennsylvania 16802, United States 7

State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking

University, Beijing 100871, China 8Materials

Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332- 0245,

USA KEYWORDS: MoS2, defect engineering, ion beam, PL, Raman

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ABSTRACT: Transition metal dichalcogenides (TMDs) have attracted much attention due to their promising optical, electronic, magnetic, and catalytic properties. Engineering the defects in TMDs represents an effective way to achieve novel functionalities and superior performance of TMD devices. However, it remains a significant challenge to create defects in TMDs in a controllable manner or to correlate the nature of defects with their functionalities. In this work, taking singlelayer MoS2 as a model system, defects with controlled densities are generated by 500 keV Au irradiation with different ion fluences, and the generated defects are mostly S vacancies. We further show that the defects introduced by ion irradiation can significantly affect the properties of the single-layer MoS2, leading to considerable changes in its photoluminescence characteristics and electrocatalytic behavior. As the defect density increases, the characteristic photoluminescence peak of MoS2 first blueshifts and then redshifts, which is due to the electron transfer from MoS2 to the absorbed O2 at the defect sites. The generation of the defects can also strongly improve the hydrogen evolution reaction activity of MoS2, attributed to the modified adsorption of atomic hydrogen at the defects.

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INTRODUCTION Molybdenum disulfide (MoS2), as a representative two-dimensional (2D) transition metal

dichalcogenide (TMD), has attracted tremendous attention in recent years due to its promising applications in electronic transistors,1-3 flexible and transparent displays,4-5 optoelectronic devices,6-8 sensors

9-12

and energy applications.13-15 Defects, such as vacancies, adatoms, edges,

grain boundaries, and substitutional atoms, are widely observed in the MoS2 structure, and they have shown critical impact on the magnetic16, electronic17, optical18, and catalytic properties.19-21 Manipulating the defects in MoS2 presents an effective way to obtain novel functionalities and


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superior device performance.19-20, 22-24 For examples, ferromagnetism was observed in defective MoS2 due to the presence of single vacancies and vacancy clusters.16 Gas sensors based on a single-layer MoS2 with excessive defects have shown much better sensitivity due to the modified electronic structure.25-26 Despite many successful examples of defect engineering in MoS2, the fundamental mechanisms of the performance enhancement, nevertheless, are still elusive. For example, while the mechanism for the enhanced hydrogen-evolution-reaction (HER) activity by S-vacancies in the MoS2 basal plane has been studied both experimentally and theoretically,14, 20-21, 27 the impacts from other types of defects (such as Mo-vacancies or Mo and S-vacancy clusters) are not yet fully understood. The impact of intrinsic defects, such as single-S vacancies, double-S vacancies, and grain boundaries, on the conductivity of CVD grown MoS2, remains elusive.28-30 Besides the fundamental understanding of the defects, another challenge is to develop an effective protocol for precisely tailoring the type, position, and density of the defects in MoS2 in order for rational design of new functional materials via defect engineering. A number of techniques have been developed to create defects in MoS2, such as thermal annealing21, plasma treatment,27 chemical functionalization,31 electron beam irradiation,26, 32 and ion beam irradiation.18, 33-37 Among these techniques, ion beam irradiation can offer a wide range of controllability over the defect type and density, by controlling the irradiation parameters such as the type, energy, and fluence of the incident ions. The interaction between an energetic ion and a solid is a non-equilibrium process, therefore it can produce defects with a variety of types and distributions, compared to those generated under a thermodynamically equilibrium condition.19, 38-39 In this work, we introduced defects with different densities in a single-layer (SL) MoS2 in a controlled manner by Au ion beam irradiation, and we systematically investigated the impact of


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the defects on the properties of MoS2. We demonstrated that the defects with controllable density could be successfully formed in MoS2 layers by 500 keV Au+ ion irradiation with different ion fluences,

as

confirmed

by

X-ray

photoelectron

spectroscopy

(XPS),

Raman

and

photoluminescence (PL) spectroscopies. As the defect density increased, the PL peak position was found to first blueshift and then redshift, which was likely due to the facilitated electron transfer from the MoS2 layer to the adsorbed O2 molecule at the defect sites. Further, we found that the HER activity of MoS2 continuously increased by introducing more defects into the basal plane. Our results showed that the ion beam irradiation is an effective approach to controllably generate defects in single-layer MoS2. These defects can modify the interaction of MoS2 with the environment, leading to significant changes in the light emission characteristics and electrocatalytic performance. The knowledge gained in this work can be applied to other TMDs, and therefore is highly beneficial for the rational design of new functional materials.

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RESULTS AND DISCCUSSION SL MoS2 was prepared on Si/SiO2 substrates by chemical vapor deposition (CVD) using

MoO3 and sulfur powders as precursors (Figure 1a). Figure 1b shows an optical image of the obtained MoS2 flakes, which were triangular in shape with an edge length of approximately 13 μm. The thickness of the MoS2 flakes was approximately 0.7 nm, as measured by atomic force microscopy (AFM) (Figure 1c), and this number was consistent with the previous works of SL MoS2.40 Raman and photoluminescence (PL) spectra were taken on these MoS2 flakes. As shown in Figure 1d, the Raman spectrum contains two peaks corresponding to the out-of-plane vibration of the S atoms (A1’ mode at ~402.3 cm-1) and the in-plane vibration of the Mo and S atoms (E’ mode at ~384.8 cm-1) of SL MoS2.40 The PL spectrum (Figure 1e) shows a strong peak at ~1.8


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eV, which is a typical feature of SL MoS2.41 These results indicate that the CVD MoS2 was of high-quality, which therefore could be used as a model system to study the impact of the defects on the properties of MoS2.

Figure 1. (a) Schematic illustration of the CVD process to grow SL MoS2 flakes. (b) Optical image of the as-grown MoS2 flakes. (c) AFM line-trace height profile across an edge of a CVD MoS2 flake. Inset shows the associated AFM image of the MoS2 flake, where the trace line is indicated. Scale bar: 1 μm. (d) Raman and (e) PL spectra of the CVD MoS2 flakes.


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Using XPS, Raman and PL spectroscopies as follows, we demonstrate that the defects with controllable density could be introduced into the SL MoS2 model system by heavy ion irradiation. The irradiation experiment was carried out using 500 keV Au+ ions with various ion fluences ranging from 5×1011 to 1×1014 ions/cm2. The average distance between neighboring ion impact sites (LD) could be estimated as LD=1/ 𝜎, where σ is the ion fluence. Assuming that each incident ion produced a defect site in MoS2, LD was then correlated with the spacing between the defect sites in the irradiated MoS2. Figure 2a shows the representative XPS spectra of the Mo 3d and S 2p core-level emissions for a pristine as-grown MoS2 sample and a sample subjected to ion irradiation with a fluence of 1×1013 ions/cm2. We extracted the S:Mo atomic ratios of the irradiated samples by fitting their peaks in XPS spectra. As shown in Figure 2b, a clear decreasing trend of the S:Mo ratio was observed with the increase of fluence (smaller LD), and, in particular, the S:Mo ratio reduced from 2 for the pristine sample to 1.67 for the sample subjected to 5×1013 ions/cm2 ion irradiation (i.e., LD = 1.414 nm). These XPS results indicate that the defects in MoS2 were mainly S vacancies, and MoS2 tended to lose S more readily than Mo atoms upon the ion irradiation, which is consistent with previous reports.19, 38 In addition, no Au peaks were observed by XPS (Figure S1), which suggests negligible doping of Au in the MoS2 layer.


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Figure 2. a) Mo 3d, S 2s and S 2p XPS spectra of MoS2 in its pristine state and after the irradiation of 500 keV Au+ ions with an ion fluence of 1×1013 ions/cm2. Gray symbols represent the experimental data, and solid lines are the fitting results. b) The S:Mo atomic ratio as a function of ion fluence and LD. The presence of the defects with controllable density in MoS2 after irradiation was further confirmed by Raman spectra, which were taken at room temperature in air using a confocal microscope system with a green (532 nm) laser.36 Figure 3a,b show the variation of the Raman spectra with respect to ion fluence, where the spectra were normalized by the A1’ peak of the SL MoS2. For the irradiation with a low ion fluence (LD>10 nm), both the A1’ and E’ peaks shifted toward higher wavenumber. Such Raman shift is consistent with the presence of compressive strain in MoS2, 42 and the compressive strain could be due to the creation of S vacancies as reported previously.43-44 As the ion fluence increased beyond 1×1012 ions/cm2 (i.e., LD

Defect Engineering in Single-Layer MoS2 Using Heavy Ion Irradiation

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