⇦Chinese Physics Letters, 2021, Vol. 38, No. 5, Article code 057305 Interfacial Charge Transfer Induced Electronic Property Tuning of MoS$_{2}$ by Molecular Functionalization Si-Han Zhou (周思含), Chun-Wei Zhou (周春伟), Xiang-Dong Yang (杨向东), Yang Li (李阳), Jian-Qiang Zhong (钟建强)*, and Hong-Ying Mao (毛宏颖)* Affiliations Department of Physics, Hangzhou Normal University, Hangzhou 311121, China Received 25 November 2020; accepted 9 March 2021; published online 2 May 2021 Supported by the National Natural Science Foundation of China (Grant No. 22002031), the Natural Science Foundation of Zhejiang Province (Grant No. LY18F010019), and the Innovation Project in Hangzhou for Returned Scholar.
*Corresponding authors. Email: zhong@hznu.edu.cn; phymaohy@hznu.edu.cn Citation Text: Zhou S H, Zhou C W, Yang X D, Li Y, and Zhong J Q et al. 2021 Chin. Phys. Lett. 38 057305    Abstract The modulation of electrical properties of MoS$_{2}$ has attracted extensive research interest because of its potential applications in electronic and optoelectronic devices. Herein, interfacial charge transfer induced electronic property tuning of MoS$_{2}$ are investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy measurements. A downward band-bending of MoS$_{2}$-related electronic states along with the decreasing work function, which are induced by the electron transfer from Cs overlayers to MoS$_{2}$, is observed after the functionalization of MoS$_{2}$ with Cs, leading to n-type doping. Meanwhile, when MoS$_{2}$ is modified with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane ($F_{4}$-TCNQ), an upward band-bending of MoS$_{2}$-related electronic states along with the increasing work function is observed at the interfaces. This is attributed to the electron depletion within MoS$_{2}$ due to the strong electron withdrawing property of $F_{4}$-TCNQ, indicating p-type doping of MoS$_{2}$. Our findings reveal that surface transfer doping is an effective approach for electronic property tuning of MoS$_{2}$ and paves the way to optimize its performance in electronic and optoelectronic devices. DOI:10.1088/0256-307X/38/5/057305 © 2021 Chinese Physics Society Article Text Since the discovery of their novel and intriguing properties, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted extensive attention because of their potential applications in optoelectronic devices, field effect transistors (FETs), and electrocatalysts,[1–11] among which MoS$_{2}$ is one of the most promising candidates for both fundamental investigations and technological applications. The MoS$_{2}$ crystal consists of a hexagonally packed layer of Mo atoms, which is sandwiched between two layers of S atoms.[12] Because of the strong intra-layer interaction within molecular layers and the weak inter-layer interaction between molecular layers, single-layer, as well as few-layer, MoS$_{2}$ can be isolated by mechanical, liquid phase and intercalation-assisted exfoliations.[13–15] Moreover, bottom-up approaches for controlled synthesis of MoS$_{2}$ have also been realized by chemical vapor deposition.[16–18] Considering the applications of MoS$_{2}$ in electronic and optoelectronic devices, the modulation of its electronic band structure is of critical importance.[12] Dimension tuning along the $z$ direction is an effective approach to modulate the electronic band structure of MoS$_{2}$. Bulk MoS$_{2}$ is of semiconductor with an indirect band gap of 1.2 eV. By thinning bulk MoS$_{2}$ to an atomic monolayer, a crossover from indirect to direct band gap can be achieved. Meanwhile, the band gap increases to 1.8 eV due to quantum confinement effects.[19–21] Intercalation tuning is another promising approach that has been developed in recent years. The most commonly used intercalants include alkali metal and $3d$ transition metal atoms.[22] The electronic properties of MoS$_{2}$ are effectively tuned because of the charge transfer between the intercalants and chalcogenide layers. Finally, the electronic modulation of MoS$_{2}$ can also be realized by heterojunction engineering, especially in controlling its carrier density without sacrificing the carrier mobility.[23–25] Tunable photoluminescence of monolayer MoS$_{2}$ has been achieved by choosing n-type or p-type dopants, and the enhanced photoluminescence was attributed to the switch between exciton photoluminescence and trion photoluminescence, which depends on the carrier density in inmonolayer MoS$_{2}$.[23] Surface functionalization is a promising approach for electronic property tuning of MoS$_{2}$. Electron transfer from alkali metals to a single crystal MoS$_{2}$ has been observed along with supra-valence electrons in oxidation reactions.[26–28] Charge carrier modulation within MoS$_{2}$ has been realized via molecular decoration with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane ($F_{4}$-TCNQ), which leads to enhanced device performance for MoS$_{2}$-based optoelectronic devices and gas sensors.[25,29,30] However, most of these studies were focused on electrical characterization or photoluminescence measurements. A comprehensive understanding from the point view of energy level alignments at the interface between surface functionalization overlayers and MoS$_{2}$ is still highly desired. In this Letter, interfacial charge transfer induced electronic property tuning of MoS$_{2}$ is realized using Cs and $F_{4}$-TCNQ, respectively. As revealed by in situ x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), the Mo $3d_{5/2}$ and S $2p_{3/2}$ core levels shift to high binding energy part when MoS$_{2}$ is functionalized with Cs. Meanwhile, its work function (WF) decreases from 4.50 eV to 2.72 eV as compared to pristine MoS$_{2}$. As a result, the Fermi level of MoS$_{2}$ shifts upward to its conduction band minimum (CBM) because of the electron transfer from Cs to MoS$_{2}$ at the interface. In the case of $F_{4}$-TCNQ modified MoS$_{2}$, interfacial electron transfer takes place under the opposite direction. Because of the strong electron withdrawing property, spontaneous electron transfer from MoS$_{2}$ to $F_{4}$-TCNQ occurs, which leads to p-type doping of MoS$_{2}$. This is corroborated by an upward band-bending of MoS$_{2}$-related electronic states along with the increasing work function.
cpl-38-5-057305-fig1.png
Fig. 1. XPS core level spectra of (a) Mo $3d$, (b) S $2p$, (c) Cs $3d$, and (d) UPS spectra at the low kinetic energy region with the increasing coverage of Cs on bulk MoS$_{2}$.
Using a multifunctional ultrahigh vacuum (UHV) VT-SPM system (Omicron Instruments for Surface Science), in situ UPS and XPS measurements were performed to investigate n- and p-type doping of MoS$_{2}$ by molecular functionalization. He I (21.2 eV) was used as the excitation source when UPS measurements were carried out. The sample WF was determined by the linear extrapolation at the low kinetic energy region of UPS spectra and a $-5$ V sample bias was applied for WF measurements. During UPS measurements, the base pressure in the analysis chamber was better than $\sim $$3\times 10^{-8}$ mbar. XPS measurements were carried out with an Al $K_\alpha$ source (1486.6 eV). A sputtered clean gold sample was used to calibrate the binding energy for all XPS spectra. The bulk MoS$_{2}$ (HF-Kejing) was peeled with scotch tape to reveal a fresh surface before annealing in the preparation chamber at 200℃ overnight helpful to remove surface adsorbates and other contaminants. The morphology of bulk MoS$_{2}$ (shown in Fig. S1 in the Supplementary Material) was measured by an atomic force microscope (AFM, Being Nano-Instruments CSPM 5500). A commercial SAES Cs getter source was used for deposition of Cs on MoS$_{2}$ and the deposition rate was calibrated by a quartz crystal microbalance (QCM). After thoroughly degassing, perylenetetracarboxylic dianhydride (PTCDA) and $F_{4}$-TCNQ (Sigma-Aldrich) were deposited onto MoS$_{2}$ by thermal deposition methods using a Knudsen cell when the base pressure in the growth chamber was better than $1\times 10^{-8}$ mbar. The nominal thicknesses of PTCDA and $F_{4}$-TCNQ were monitored by the QCM and confirmed by the attenuation of Mo $3d$ XPS core levels after the deposition of organic molecules. The n-type doping of MoS$_{2}$ has been investigated using in situ UPS and XPS measurements. Figures 1(a)–1(c) show the XPS core level spectra of Mo $3d$, S $2p$, and Cs $3d$ with the increasing coverage of Cs on bulk MoS$_{2}$. For pristine MoS$_{2}$, the Mo $3d_{5/2}$ peak locates at 229.38 eV, and the full width at half maximum (FWHM) is 1.18 eV. Upon the functionalization of MoS$_{2}$ with Cs, the binding energy of the Mo $3d_{5/2}$ core level increases gradually. As shown in Fig. 1(a), when the thickness of Cs increases to 2.0 nm, the binding energy of the Mo $3d_{5/2}$ peak increases by 0.28 eV to 229.66 eV. Consistent with Mo $3d_{5/2}$ core level, the binding energy of the S $2p_{3/2}$ peak increases from 162.18 eV to 162.54 eV, corresponding to a 0.36 eV shift to high binding energy part. Meanwhile, the FWHMs of Mo $3d_{5/2}$ and S $2p_{3/2}$ core level peaks do not change after Cs functionalization, indicating that it is the doping effect rather than chemical reaction that is responsible for the downward band-bending in MoS$_{2}$. Although the binding energy of Cs $3d$ core level does not change [Fig. 1(c)], there is a significant shift of the secondary electron cut-off in the low kinetic energy region upon the deposition of Cs onto MoS$_{2}$. The WF for pristine MoS$_{2}$ is 4.50 eV, and it decreases significantly to 2.72 eV when the nominal thickness of Cs reaches 2.0 nm. The low WF of Cs leads to the spontaneous electron transfer from Cs to MoS$_{2}$ upon contact. Consequently, the Fermi level of MoS$_{2}$ shifts upward toward its CBM, leading to the downward band-bending in MoS$_{2}$. The observed large vacuum level shift (WF decrease) of 1.78 eV derives from the convolution of downward band-bending (0.28 eV) in MoS$_{2}$ and the interfacial dipole formed at the Cs/MoS$_{2}$ interface, as plotted in Fig. S2(a). It is worth noting that the decreasing WF, together with the downward band-bending Mo $3d$ and S $2p$, is more obvious in the interface region when the coverage of Cs is 0.2 nm, and it saturates quickly when the coverage of Cs increases to 1.0 nm. This indicates that such an electron transfer from Cs to MoS$_{2}$ is more pronounced within the submonolayer coverage of surface functionalization overlayers. Finally, we need to mention that the binding energy of Cs $3d$ should shift toward high binding energy part with the increasing coverage of Cs since electron transfer from Cs to MoS$_{2}$ occurs upon the formation of Cs/MoS$_{2}$ interface. Although these binding energy shifts of dopants are not observed in the present study, a similar phenomenon is observed for surface transfer hole doping of epitaxial graphene by MoO$_{3}$ and more investigation should be carried out in future studies.[31]
cpl-38-5-057305-fig2.png
Fig. 2. XPS core level spectra of (a) Mo $3d$, (b) S $2p$, (c) C $1s$, (d) O $1s$, and (e) UPS spectra at the low kinetic energy region with the increasing coverage of PTCDA on bulk MoS$_{2}$. The inset of Fig. 2(c) shows the molecular structure of PTCDA.
The charge transfer at the interface between MoS$_{2}$ and dopants is of great importance in determining the doping efficiency. As we have discussed above, the low WF of Cs leads to the electron transfer from Cs to MoS$_{2}$, which results in the n-type doping of MoS$_{2}$. A commonly used molecule in organic optoelectronic devices, PTCDA has been chosen to investigate the interfacial charge transfer induced doping effect in MoS$_{2}$. Unlike the interface between Cs and MoS$_{2}$ where interfacial charge transfer is facilitated due to low WF of Cs, the Fermi level of MoS$_{2}$ locates in the band gap of PTCDA, which could inhibit the charge transfer after the formation of the PTCDA/MoS$_{2}$ interface.[32] As presented in Figs. 2(a) and 2(b), the binding energies of Mo $3d_{5/2}$ and S $2p_{3/2}$ peaks do not change after the modification of MoS$_{2}$ with PTCDA at the increasing coverage from 0.2 nm to 2.0 nm. It is generally believed that n-type doping leads to a downward band-bending in MoS$_{2}$, whereas p-type doping results in an upward band-bending in MoS$_{2}$. Since there are no binding energy shifts for both Mo $3d_{5/2}$ and S $2p_{3/2}$ core levels after the functionalization of MoS$_{2}$ with PTCDA overlayers, we conclude that no doping effect occurs in PTCDA/MoS$_{2}$ in view of the diagram of interfacial energy levels. Moreover, XPS results also show the unchanged C $1s$ and O $1s$ core levels [Figs. 2(c) and 2(d)] along with the unchanged WF [Fig. 2(e)]. There are two peaks for C $1s$ core level in PTCDA/MoS$_{2}$, as shown in Fig. 2(c), the main component at 285.28 eV is attributed to carbon atoms from the perylene core, and the weak component at 289.08 eV derives from carbon atoms from the carbonyl groups.[33–35] In the case of O $1s$ core level shown in Fig. 2(d), two peaks at 532.08 eV and 534.04 eV are attributed to anhydride and carboxylic oxygen atoms, respectively. Consistent with Mo $3d_{5/2}$ or S $2p_{3/2}$ core levels, no binding energy shifts are observed for both C $1s$ and O $1s$ core levels. In addition, the WF for PTCDA modified MoS$_{2}$ [Fig. 2(e)] is 4.46 eV, which is nearly the same as the WF of pristine MoS$_{2}$. Consequently, the Fermi level of MoS$_{2}$ locates between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PTCDA. As a result, the charge transfer at the interface between PTCDA and MoS$_{2}$ is energetically unfavorable, and hence no doping effect is observed. In the case of chemical vapor deposition grown MoS$_{2}$ films and mechanical exfoliated MoS$_{2}$ flakes, unintentional electron doping is usually observed because of the gap states below the conduction band edge, which are introduced by the sulfur vacancies. This is also confirmed by our UPS results. As shown in Fig. S3, the edge of the valence band maximum (VBM) of bulk MoS$_{2}$ is located at 1.12 eV below the Fermi level. Considering that the band gap of bulk MoS$_{2}$ is 1.2 eV, the Fermi level locates just 0.08 eV below the edge of CBM, indicating the electron doping of pristine MoS$_{2}$. To deplete residual electrons from sulfur vacancies, surface functionalization of MoS$_{2}$ by organic molecules with strong electron withdrawing property opens up a pathway for such modulations. The electron depletion or p-type doping of MoS$_{2}$ via $F_{4}$-TCNQ overlayers is investigated by in situ UPS and XPS measurements. Due to the strong electron withdrawing property of $F_{4}$-TCNQ, electron transfer from MoS$_{2}$ to $F_{4}$-TCNQ takes place after the formation of $F_{4}$-TCNQ/MoS$_{2}$ interfaces, where the Fermi level of MoS$_{2}$ moves downward to its VBM, leading to the upward band-bending in MoS$_{2}$. This upward band-bending at the interface is confirmed by our XPS results.
cpl-38-5-057305-fig3.png
Fig. 3. XPS core level spectra of (a) Mo $3d$, (b) S $2p$, (c) C $1s$, (d) F $1s$, (e) N $1s$, and (f) UPS spectra at the low kinetic energy region with the increasing coverage of $F_{4}$-TCNQ on bulk MoS$_{2}$. The inset of Fig. 3(c) shows the molecular structure of $F_{4}$-TCNQ.
Figures 3(a) and 3(b) present the XPS core level spectra of Mo $3d$ and S $2p$ before and after the modification of MoS$_{2}$ with $F_{4}$-TCNQ. The Mo $3d_{5/2}$ peak shifts to 229.00 eV after the thickness of $F_{4}$-TCNQ increases to 2.0 nm, corresponding to a 0.42 eV upward band-bending at the interface. This upward band-bending is also observed in the case of S $2p_{3/2}$ core level, where the S $2p_{3/2}$ peak shows a 0.46 eV shift toward to the low binding energy part. Similar to the case of Cs modified MoS$_{2}$, there are no obvious binding energy shifts for the characteristic C $1s$, F $1s$, and N $1s$ peaks in $F_{4}$-TCNQ, as shown in Figs. 3(c)–3(e). However, the WF of MoS$_{2}$ increases from 4.52 eV to 5.36 eV after $F_{4}$-TCNQ modification, as illustrated by the significant shift of the secondary electron cut-off toward high kinetic energy part [Fig. 3(f)]. Similar to the case of Cs modified MoS$_{2}$, the large vacuum level shift (WF increase) of 0.84 eV derives from the convolution of upward band-bending (0.42 eV) in MoS$_{2}$ and the interfacial dipole formed at the $F_{4}$-TCNQ/MoS$_{2}$ interface [Fig. S2(b)]. The Fermi level of MoS$_{2}$ shifts downward due to the electron depletion upon the formation of $F_{4}$-TCNQ/MoS$_{2}$ interfaces. Moreover, it is worth noting that a 0.44 eV upward binding energy shift of Mo $3p$ core level [Fig. 3(e)] is also observed after the functionalization of MoS$_{2}$ by $F_{4}$-TCNQ, which further corroborates the p-type doping of MoS$_{2}$ by $F_{4}$-TCNQ overlayers. A schematic diagram of energy levels of Cs doped, pristine, and $F_{4}$-TCNQ doped MoS$_{2}$ is shown in Fig. 4. Charge transfer at the interfaces between dopants and MoS$_{2}$ plays a critical role for electrical property tuning of MoS$_{2}$. In the case of pristine MoS$_{2}$, the Fermi level locates just below the CBM because of the unintentional electron doping deriving from the existence of sulfur vacancies. When low WF Cs has been exploited as surface functionalization overlayers, electron transfer from Cs to MoS$_{2}$ occurs, leading to an n-type doping of MoS$_{2}$. Meanwhile, when $F_{4}$-TCNQ is used to functionalize MoS$_{2}$, electron transfer from MoS$_{2}$ to $F_{4}$-TCNQ occurs because of the strong electron withdrawing property of $F_{4}$-TCNQ, leading to a p-type doping of MoS$_{2}$.
cpl-38-5-057305-fig4.png
Fig. 4. Schematic diagram of energy levels for Cs doped, pristine, and $F_{4}$-TCNQ doped MoS$_{2}$.
In summary, tuning the electronic properties of MoS$_{2}$ has been realized by Cs and $F_{4}$-TCNQ overlayers, which is confirmed by in situ UPS and XPS measurements. After the modification of MoS$_{2}$ with 2.0 nm Cs, a downward band-bending of MoS$_{2}$-related electronic states at the interface has been observed, corresponding to the n-type doping of MoS$_{2}$. When 2.0 nm $F_{4}$-TCNQ has been deposited onto MoS$_{2}$, an upward band-bending of MoS$_{2}$-related electronic states has been observed, corresponding to the p-type doping of MoS$_{2}$. Our finding indicates that charge transfer at the interfaces between dopants and MoS$_{2}$ is of significant importance for the electronic property tuning of MoS$_{2}$, and we propose a practical strategy for potential optimization of MoS$_{2}$-related electronic and optoelectronic devices. References The chemistry of two-dimensional layered transition metal dichalcogenide nanosheetsElectronics and optoelectronics of two-dimensional transition metal dichalcogenidesSingle-Layer MoS 2 PhototransistorsFew-layer molybdenum disulfide transistors and circuits for high-speed flexible electronicsA Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen GenerationUltrasensitive photodetectors based on monolayer MoS2Optoelectronic properties of laser-beam-patterned few-layer lateral MoS 2 Schottky junctionsChemical Vapor Deposition Growth of Large-Area Monolayer MoS 2 and Fabrication of Relevant Back-Gated Transistor * .Two-dimensional transition metal dichalcogenide nanosheet-based compositesPhonon Confinement Effect in Two-dimensional Nanocrystallites of Monolayer MoS 2 to Probe Phonon Dispersion Trends Away from Brillouin-Zone CenterPolymer-Decorated 2D MoS 2 Synaptic Transistors for Biological Bipolar Metaplasticities EmulationPhysical and chemical tuning of two-dimensional transition metal dichalcogenidesTwo-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challengesElectrical Characteristics of Molybdenum Disulfide Flakes Produced by Liquid Exfoliation1T MoS2 nanosheets with extraordinary sodium storage properties via thermal-driven ion intercalation assisted exfoliation of bulky MoS2Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniquesCVD Grown MoS 2 Nanoribbons on MoS 2 Covered Sapphire(0001) Without CatalystsWafer-scale MoS2 thin layers prepared by MoO3 sulfurizationEmerging Photoluminescence in Monolayer MoS 2Electronic structure of two-dimensional crystals from ab initio theoryAtomically Thin MoS 2 : A New Direct-Gap SemiconductorChemical Stabilization of 1T′ Phase Transition Metal Dichalcogenides with Giant Optical Kerr NonlinearityTunable Photoluminescence of Monolayer MoS 2 via Chemical DopingModulating electronic transport properties of MoS 2 field effect transistor by surface overlayersCharge Transfer within the F 4 TCNQ-MoS 2 van der Waals Interface: Toward Electrical Properties Tuning and Gas Sensing ApplicationIE=edge" http-equiv="X-UA-Compatible"> The interaction of Cs and O2 on the basal plane of MoS2The effect of MoS2 structural changes on the adsorption of CsControl of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 MonolayersLarge modulation of mobile carriers within MoS2 by decoration of molecular dopants to enhance its gas sensingSurface transfer hole doping of epitaxial graphene using MoO3 thin filmElectronic polarization at surfaces and thin films of organic molecular crystals: PTCDAInvestigation of the growth of PTCDA on Cu(): an STM studyPost deposition purification of PTCDA thin filmsMolecular-Scale Investigation of the Thermal and Chemical Stability of Monolayer PTCDA on Cu(111) and Cu(110)
[1] Chhowalla M, Shin H S, Eda G, Li L J, Loh K P, and Zhang H 2013 Nat. Chem. 5 263
[2] Wang Q H, Kalantar-Zadeh K, Kis A, Coleman J N, and Strano M S 2012 Nat. Nanotechnol. 7 699
[3] Yin Z Y, Li H, Li H, Jiang L, Shi Y M, Sun Y H, Lu G, Zhang Q, Chen X D, and Zhang H 2012 ACS Nano 6 74
[4] Cheng R, Jiang S, Chen Y, Liu Y, Weiss N, Cheng H C, Wu H, Huang Y, and Duan X F 2014 Nat. Commun. 5 5143
[5] Karunadasa H I, Montalvo E, Sun Y J, Majda M, Long J R, and Chang C J 2012 Science 335 698
[6] Lopez-Sanchez O, Lembke D, Kayci M, Radenovic A, and Kis A 2013 Nat. Nanotechnol. 8 497
[7] Nagamine Y, Sato J, Qian Y, Inoue T, Nakamura T, Maruyama S, Katsumoto S, and Haruyama J 2020 Appl. Phys. Lett. 117 043101
[8] Chen J Y, Liu L, Li C X, and Xu J P 2019 Chin. Phys. Lett. 36 037301
[9] Tan C L and Zhang H 2015 Chem. Soc. Rev. 44 2713
[10] Shi W, Zhang X, Li X L, Qiao X F, Wu J B, Zhang J, and Tan P H 2016 Chin. Phys. Lett. 33 057801
[11] Zhao Y H, Liu B, Yang J L, He J, and Jiang J 2020 Chin. Phys. Lett. 37 088501
[12] Wang H T, Yuan H T, Hong S S, Li Y B, and Cui Y 2015 Chem. Soc. Rev. 44 2664
[13] Duan X D, Wang C, Pan A L, Yu R Q, and Duan X F 2015 Chem. Soc. Rev. 44 8859
[14] L, Kim H Y, Lotya M, Coleman J N, Kim G T, and Duesberg G S 2011 Adv. Mater. 23 4178
[15] Sun D, Huang D, Wang H Y, Xu G L, Zhang X Y, Zhang R, Tang Y G, Abd E H D, Alshitari W, Al-Bogami A S, Amine K, and Shao M H 2019 Nano Energy 61 361
[16] Shi Y M, Li H N, and Li L J 2015 Chem. Soc. Rev. 44 2744
[17] Wu D, Shi J, Zheng X M, Liu J X, Dou W D, Gao Y L, Yuan X M, Ouyang F P, and Huang H 2019 Phys. Status Solidi RRL 13 1900063
[18] Lin Y C, Zhang W J, Huang J K, Liu K K, Lee Y H, Liang C T, Chu C W, and Li L J 2012 Nanoscale 4 6637
[19] Splendiani A, Sun L, Zhang Y B, Li T S, Kim J, Chim C Y, Galli G, and Wang F 2010 Nano Lett. 10 1271
[20] Lebègue S and Eriksson O 2009 Phys. Rev. B 79 115409
[21] Mak K F, Lee C, Hone J, Shan J, and Heinz T F 2010 Phys. Rev. Lett. 105 136805
[22] Tan S J R, Abdelwahab I, Ding Z J, Zhao X X, YangT S, Loke G Z J, Lin H, Verzhbitskiy I, Poh S M, Xu H, Nai C T, Zhou W, Eda G, Jia B H, and Loh K P 2017 J. Am. Chem. Soc. 139 2504
[23] Mouri S, Miyauchi Y, and Matsuda K 2013 Nano Lett. 13 5944
[24] Lin J D, Zhong J Q, Zhong S, Li H, Zhang H, and Chen W 2013 Appl. Phys. Lett. 103 063109
[25] Wang J W, Ji Z Y, Yang G H, Chuai X C, Liu F J, Z, Lu C Y, Wei W, Shi X W, Niu J B, Wang L, Wang H, Chen J Z, Lu N D, Jiang C, Li L, and Liu M 2018 Adv. Funct. Mater. 28 1806244
[26] Park K T and Kong J 2002 Top. Catal. 18 175 [27] Kennou S, Ladas S, and Papageorgopoulos C A 1985 Surf. Sci. 164 290
[28] Ladas S, Foulias S D, and Papageorgopoulos C A 1984 Solid State Commun. 52 543
[29] Hu P, Ye J, He X X, Du D Z, Zhang K, Wang X Z, Xiong Q H, Liu Z, Jiang H, and Kloc C 2016 Sci. Rep. 6 24105
[30] Chen Y Y, Liu F J, Wang J W, Zhang Y, Fan Y, Liu M, Zhao Y F, Wang G T, and Jiang C 2020 Appl. Surf. Sci. 527 146709
[31] Chen Z Y, Santoso I, Wang R, Xie L F, Mao H Y, Huang H, Wang Y Z, Gao X Y, Chen Z K, Ma D G, Wee A T S, and Chen W 2010 Appl. Phys. Lett. 96 213104
[32] Tsiper E V, Soos Z G, Gao W, and Kahn A 2002 Chem. Phys. Lett. 360 47
[33] Stöhr M, Gabriel M, and Möller R 2002 Surf. Sci. 507 330
[34] Wüsten J, Ertl T, Lach S, and Ziegler C 2005 Appl. Surf. Sci. 252 104
[35] Gu C D, Zhang J L, Sun S, Lian X, Ma Z R, Mao H Y, Guo L, Wang Y P, and Chen W 2020 ACS Appl. Mater. Interfces 12 22327