Basal-Plane Ligand Functionalization on ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Article

Basal Plane Ligand Functionalization on Semiconducting 2H-MoS2 Monolayers Qi Ding, Kyle J. Czech, Yuzhou Zhao, Jianyuan Zhai, Robert J Hamers, John C. Wright, and Song Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01262 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Basal Plane Ligand Functionalization on Semiconducting 2H-MoS2 Monolayers Qi Ding, Kyle J. Czech, Yuzhou Zhao, Jianyuan Zhai, Robert J. Hamers, John C. Wright, Song Jin* Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States *

Corresponding author: [email protected].

ABSTRACT Molybdenum disulfide (MoS2) is a two-dimensional material promising for electronic, optical, and catalytic applications. To fully harness its potential, functionalization is essential to control its properties. However, MoS2 functionalization has been mostly limited to either 1T-phase MoS2 or the edges of 2H-phase MoS2, and the chemistry of covalent functionalization on the basal plane of 2H-MoS2 is poorly understood. Here we report a facile approach to covalently functionalize chemical vapor deposition (CVD) grown 2H-MoS2 monolayers (MLs), as well as mechanically exfoliated MoS2, via thiol conjugation at sulfur vacancies on the basal plane. Thorough characterization confirmed the functionalization by thiol molecules on MoS2 MLs and we experimentally proved that sulfur vacancies in MoS2 MLs play a key role in the functionalization of basal planes. By controlling the amount of sulfur vacancies via sulfur annealing, the degree of MoS2 functionalization was effectively tuned. Because thiol conjugation partially repairs or passivates sulfur vacancies, enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis were observed for functionalized MoS2. 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Moreover, such functionalization can be utilized for making MoS2-based heterostructures, an example of which was demonstrated using a dithiol molecule to link MoS2 layers and PbSe quantum dots. These results provide new understanding and insights on the surface chemistry of MoS2 and open up more opportunities for MoS2 MLs with well-controlled properties and broader applications. KEYWORDS: MoS2, monolayer, ligand functionalization, basal plane, sulfur vacancy

1. INTRODUCTION Two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) are a family of materials that have attracted widespread attention due to their potential applications in electronics, optoelectronics, sensing, energy storage, and catalysis.1-6 MoS2 has been one of the most studied TMDCs and acts as an excellent model system to explore the chemistry of 2D TMDCs. It consists of individual sandwiched S−Mo−S layers held together by weak van der Waals interactions and exhibits three common polymorphs, 2H, 3R and 1T.7 Unique optical and electronic properties emerge when bulk MoS2 is thinned to a single layer, making MoS2 monolayers (MLs) particularly attractive. Photoluminescence (PL) is observed in MoS2 MLs due to quantum confinement and the associated change from indirect band gap (1.3 eV) to direct band gap (1.8 eV),8 which generates possibilities for many optoelectronic applications such as light-emitting diodes and photodetectors.9-12 Field effect transistors (FETs) based on MoS2 monolayers were also demonstrated to have high mobilities and on−off current ratios.13 Despite these promising material properties, the lack of understanding and control over MoS2 surface functionalization poses limitations on its properties and applications, because 2 ACS Paragon Plus Environment

Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface functionalization can offer convenient platform to tailor the electronic, optical and catalytic properties and fully harness the capabilities of MoS2.14-19 Functionalization of 2D MoS2 materials would also facilitate their biological applications20-21 and integration into nanocomposites.14-15 MoS2 is traditionally considered rather chemically inert, especially along the basal plane that form the van der Waals gaps in the 2D layered structure.2-4, 15 Intuitively, the broken bonds at the edges of the MoS2 layers would increase the reactivity, and the edge sites of 2H-MoS2 have been shown to indeed display different catalytic and physical properties than those of the basal plane.22 Edge functionalization of 2H-MoS2 (bulk or layers) has been studied since the 1980s in order to understand and control the high density of defects and high reactivity.23-26 However, no reports have explicitly demonstrated and showed conclusive experimental evidences to support covalent functionalization on the basal planes of 2H-MoS2 monolayers. Moreover, the few studies of 2HMoS2 functionalization were focused on liquid exfoliated few-layer MoS2 nanosheets.24-28 The complex morphology and the high defect density often seen in such colloidal nanostructures prevents definitive characterization and differentiation between the edges and the basal plane. Furthermore, the chemistry of liquid exfoliated colloidal MoS2 nanosheets can be very different from the MoS2 monolayers made via chemical vapor deposition (CVD) or mechanical exfoliation that are of interest for typical device applications. In contrast to 2H-MoS2, 1T-MoS2 possesses a very different electronic structure and surface chemistry.5 It is a metastable phase of MoS2 that can be obtained by converting from the thermodynamically stable 2H-MoS2 via chemical exfoliation. Covalent functionalization on the basal planes of chemically exfoliated 1T-MoS2 nanosheets have been demonstrated by reacting them with organohalide and other molecules,29-30 which led to changes in the optoelectronic properties of the materials. Alternatively, chemically exfoliated 1TMoS2 nanosheets could also be functionalized via thiol conjugation at the edges as well as on the

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

basal planes, as many defects are generated during the harsh chemical exfoliation treatment required for the 1T-MoS2 preparation, which are believed to form covalent bonds with thiolcontaining molecules.17, 20-21, 31-34 In contrast to these studies, the basal plane surface chemistry of semiconducting 2H-MoS2 monolayers has not yet been carefully explored. There have been some studies on the changes in the properties and performance of devices made of mechanically exfoliated few-layer MoS2 upon passivation or “doping” by organic functional groups, but the surface chemistry was not clearly elucidated and the hypothesized functionalization mechanism was not confirmed.18-19 It is important to develop a fundamental understanding of the mechanisms and be able to control the surface functionalization so that we can utilize such functionalization for creating more complex nanostructures. Even though the basal plane of 2H-MoS2 is generally believed to be too inert for functionalization, it is not perfectly coordinated. Both experimental and computational studies have shown that a wide range of defect types are present and high densities of sulfur vacancies are observed on the basal plane of 2H-MoS2 monolayers derived from several methods.17, 35-40 These sulfur vacancies could be reactive centers that enable basal plane ligand conjugation on semiconducting 2H-MoS2 monolayers, providing a useful means by which to tune the electronic, optical and catalytic properties of MoS2. In this work, we investigate the surface chemistry of CVD grown 2H-MoS2 monolayers, demonstrate and experimentally confirm successful covalent functionalization of 4-fluorobenzyl mercaptan and other thiol-containing molecules on the basal plane of MoS2 MLs via thiol conjugation (Figure 1). Importantly, using the F-containing ligand as a marker, we clearly characterized the functionalized products using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. We further correlated the degree of functionalization to the amount of sulfur vacancies present on MoS2 ML basal plane. 4 ACS Paragon Plus Environment

Page 4 of 28

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


An enhanced photoluminescence (PL) response and significant reduction in catalytic activity are observed for functionalized MoS2 MLs, consistent with partial repair and/or passivation of sulfur vacancies by thiol ligand conjugation. The generality of this functionalization also permits expansion to other thiol-containing ligands with different functional groups. Furthermore, heterostructures of MoS2 and PbSe quantum dots are successfully prepared using 1,5pentanedithiol as a linker molecule. These functionalization methods open new pathways for facile synthesis of MoS2-based heterostructures.

Figure 1. Schematic of ligand conjugation on sulfur vacancies on basal plane of 2H-MoS2 monolayers.

2. RESULTS AND DISCUSSION We synthesized MoS2 monolayers on Si/SiO2 or sapphire substrates using a CVD reaction by reacting molybdenum oxide films with sulfur precursor vapor at 750 oC.41 As shown in Figure 2a, 7-15 µm, single-layer MoS2 flakes were formed on the substrate after the reaction. We chose 4-fluorobenzyl mercaptan (see Figure 1) as a representative organic molecule with a primary thiol functional group as well as a distinctive F element as a marker for MoS2 ML functionalization detection. Such F marker atom is crucial for establishing the surface functionalization by thiol 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecules because the S atoms in thiol molecules are also present in MoS2. The MoS2 MLs on Si/SiO2 substrate was soaked in neat 4-fluorobenzyl mercaptan liquid for 48 hours and then soaked in ethanol, acetone, isopropanol and water for 10 min each, and rinsed harshly with these solvents to wash off any physisorbed ligands. To examine if there is any 4-fluorobenzyl mercaptan left on the MoS2 MLs surface after washing, we first conducted FTIR on the treated MoS2 ML sample (Figure 2b red curve) in comparison with the free 4-fluorobenzyl mercaptan ligand (Figure 2b black curve). Peaks corresponding to C-H, C=C and C-F stretches were clearly observed on the treated MoS2 MLs sample. They match the peaks from 4-fluorobenzyl mercaptan, confirming the successful functionalization of the MoS2 ML surface by 4-fluorobenzyl mercaptan. Notably, as highlighted in the box of Figure 2b, the S-H stretch peak at 2550 cm-1 that appears in the 4fluorobenzyl mercaptan spectrum (black curve) was no longer observed in the functionalized MoS2 ML sample. The disappearance of S-H peak has also been observed in other thiol conjugated systems, and is commonly attributed to thiol functional group successfully conjugating to MoS2 surface with the S-H bond cleaved,20, 26, 28 providing more evidence that 4-fluorobenzyl mercaptan is conjugating with the MoS2 surface, not physisorbed on the surface. Slight changes in the FTIR spectra were observed between functionalized MoS2 ML and free ligand, a phenomenon that has also previously been observed in other thiol conjugated systems, which might be attributed to potential changes in the ligand chemical environment.20, 28


Page 6 of 28

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) A typical optical microscopy image of CVD grown MoS2 monolayers on sapphire substrate. (b) FTIR spectra of 4-fluorobenzyl mercaptan functionalized MoS2 MLs (red) in comparison to the free 4-fluorobenzyl mercaptan ligand (black). (c) High-resolution XPS of F1s region for 4-fluorobenzyl mercaptan functionalized MoS2 ML (red), fluorobenzene treated MoS2 ML (blue) and non-functionalized MoS2 ML (black). (d) Fitted high-resolution XPS of S2p regions for 4-fluorobenzyl mercaptan functionalized MoS2 MLs (red) and non-functionalized MoS2 MLs (black).

To gain more insights into the functionalization, XPS was conducted to reveal potential changes in the chemical environment of MoS2 MLs before and after functionalization. As shown in the high-resolution F1s spectra (Figure 2c), as-grown MoS2 MLs do not show any F peak (black curve), indicating there is no F contamination on MoS2 MLs substrate. After 4-fluorobenzyl mercaptan functionalization, a distinct F peak was clearly observed (red curve), confirming MoS2 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MLs surface has been successfully functionalized. A control experiment using fluorobenzene, a molecule with similar chemical structure with 4-fluorobenzyl mercaptan but without the thiol functional group, was also used for functionalization. However, as shown in the blue curve in Figure 2c, no F peak was observed in XPS, indicating the functionalization using fluorobenzene was not successful. This difference between 4-fluorobenzyl mercaptan and fluorobenzene strongly suggests that functionalization on MoS2 MLs relies on thiol functional group, without which, ligands could only be physisorbed on the surface and thus can be easily washed off. In addition to the appearance of the F peak in XPS, the S XPS peak was broadened and slightly shifted to lower binding energy after 4-fluorobenzyl mercaptan functionalization (Figure 2d). The peak deconvolution of as-synthesized MoS2 MLs reveals the presence of two S doublets, indicating two chemical environments: a major doublet at 162.7 eV and a minor doublet at 162.1 eV (all binding energy is referred to S2p3/2 peak unless noted otherwise). Free 4-fluorobenzyl mercaptan ligand is also measured and the S2p3/2 peak appears at 162.7 eV. After MoS2 ligand functionalization, it is observed the intensity of MoS2 minor doublet at 162.1 eV is significantly enhanced, which is different with the peak position of the free ligand (162.7 eV). Similar phenomenon has been observed in thiol-containing molecules self-assembling on gold surface,42 in which the emergence of peak near 162 eV is attributed to chemisorbed thiol molecules forming a covalently bound film to the gold substrate, whereas the peak near 163 eV is attributed to unbound physisorbed thiol molecules. For the MoS2 functionalization herein, it is hard to differentiate the physisorbed thiol molecules with MoS2 the major doublet from MoS2 due to their overlapping peak positions (both near 162.7 eV). Nevertheless, the clear emergence of S component at 162.1 eV (S2p3/2 peak) in the functionalized MoS2 ML sample suggests that the 4fluorobenzyl mercaptan molecules are covalently bond to MoS2 surface via thiol conjugation. 8 ACS Paragon Plus Environment

Page 8 of 28

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


With FTIR and XPS results both indicating thiol functional group conjugating to MoS2 monolayer surface, we aim to provide a more thorough investigation regarding what enables and controls the functionalization. Theoretical calculations have suggested that sulfur vacancies could be the key to thiol conjugation on the basal plane of 2H-MoS2.17,

39

Therefore, we designed

experiments to characterize and purposely tune the amount of S vacancies in 2H-MoS2 MLs, and examine if different amounts of S vacancies would lead to different degrees of functionalization. In general, annealing MoS2 in a sulfur environment has been reported to partially compensate S vacancies.43-44 We annealed the MoS2 ML sample with sulfur vapor at 550 ˚C for 2 h in a CVD chamber, and compared the amount of S vacancies before and after annealing for the same MoS2 ML sample using electron paramagnetic resonance (EPR), a technique that is sensitive to unpaired electrons and has been used to reveal the concentration of S vacancies in MoS2.45 According to literature, the signal at g = 2.00 originates from S vacancies in MoS2, with an enhanced signal suggesting increased S vacancy concentration.45 As shown in Figure 3a, after S annealing experiment, the EPR peak at g = 2.00 decreased dramatically from the black curve to the red curve, demonstrating we have successfully reduced the amount of S vacancies.

Figure 3. (a) EPR spectra for the same MoS2 ML sample measured before (black) and after (red) sulfur annealing, showing sulfur annealing caused a dramatic decrease in signal intensity. (b) High-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

resolution XPS of the F1s region for functionalized as-grown MoS2 ML (black) and functionalized annealed MoS2 ML (red) after normalization to the same Mo peak.

To investigate how the decreased S vacancy concentration affects the degree of MoS2 functionalization, we conducted the same 4-fluorobenzyl mercaptan functionalization experiment on annealed MoS2 ML. Figure 3b compares the high-resolution F1s XPS spectra for functionalized MoS2 ML (black) and functionalized annealed MoS2 ML (red) after normalization to the same Mo peak. Thus, the area of integrated F peak reflects the degree of 4-fluorobenzyl mercaptan functionalization. As expected, the sulfur annealed MoS2 ML indeed displays a much smaller F peak after functionalization than as-synthesized MoS2 ML, indicating fewer ligands and smaller degree of functionalization for annealed MoS2. This comparison confirms our hypothesis that the degree of thiol conjugation is dependent on the S vacancies present on the basal plane of MoS2 layers, and can be tuned effectively by adjusting the amount of S vacancies via S annealing. We also demonstrated that this functionalization approach could be readily applied to other thiol-containing molecules besides 4-fluorobenzyl mercaptan. For example, 5-(trifluoromethyl) pyridine-2-thiol contains a thiol functional group as well as characteristic F and N elements (see Figure S1). XPS spectra of MoS2 ML functionalized with this molecule clearly reveal the presence of F peak (Figure S1), confirming the functionalization could be successfully extended to other thiol-containing molecules. For ligands like 1,5-pentanedithol (see Figure S2) that do not contain characteristic marker elements like F and N, MoS2 ML functionalized with such molecules displayed an additional enhancement of the second S component (the S2p3/2 peak positioned near 162 eV) in the XPS S spectra (Figure S2). It is attributed to the thiol functional group conjugating


Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to MoS2 surface, consistent with what was observed for 4-fluorobenzyl mercaptan functionalized MoS2 ML (Figure 2d). Moreover, this functionalization approach can be applied to other types of MoS2, such as monolayer or few-layer MoS2 mechanically exfoliated from single crystals, and CVD grown MoS2 nanoflowers.5 Single- and multilayer MoS2 were mechanically exfoliated onto Si/SiO2 substrate, where the same 4-fluorobenzyl mercaptan molecule was used for functionalization. Figure S3 displays the distinct F1s peak that confirmed the successful ligand functionalization on mechanically exfoliated MoS2, similar to what was observed on CVD grown MoS2 MLs. We also performed the same functionalization on MoS2 nanoflower samples grown on graphite substrates synthesized via CVD.5 As shown in the FTIR spectrum (Figure S4), peaks corresponding to 4-fluorobenzyl mercaptan ligand were clearly resolved, confirming the successful functionalization of MoS2 nanoflowers. After the confirmation of ligand functionalization and the functionalization mechanism investigation, we then examined the effect of ligand functionalization on the physical and chemical properties of MoS2 MLs. Microscope photoluminescence (PL) experiments were conducted to compare the same MoS2 ML sample at the same spot before and after functionalization. Figure 4a reveals a significantly enhanced PL response for CVD MoS2 ML after 4-fluorobenzyl mercaptan functionalization. Previous work has reported using thiol chemistry to repair sulfur vacancies and improve MoS2 charge transport properties.39 Thus, it is suspected the conjugated thiol molecules that bind to the basal plane at S vacancies can repair or passivate sulfur vacancies of MoS2 ML, eliminating defect-mediated nonradiative recombination, thus resulting in enhanced PL response.46-47 Importantly, this enhancement is not only seen in 4-fluorobenzyl mercaptan functionalized MoS2 MLs, but has been consistently observed in other types of functionalized MoS2 samples. Figure 4b compares the PL responses of 1,5-pentanedithol functionalized MoS2 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

MLs before and after functionalization, and Figure 4c shows 4-fluorobenzyl mercaptan functionalized mechanically exfoliated (ME) MoS2. In all cases, a significant PL enhancement was observed, which provides additional supporting evidence for the basal plane ligand functionalization on various types of MoS2 samples and suggests the generality of such functionalization to improve photoluminescence response of MoS2.

Figure 4. Photoluminescence spectra of as-prepared MoS2 (black) and ligand functionalized MoS2 (red): (a) 4-fluorobenzyl mercaptan functionalized CVD grown MoS2; (b) 5-pentanedithiol functionalized CVD grown MoS2; (c) 4-fluorobenzyl mercaptan functionalized mechanically exfoliated (ME) MoS2, with functionalized MoS2 showing a consistent enhancement in PL response.

Besides optical and electronic applications, MoS2 is also a promising earth-abundant electrocatalyst for hydrogen evolution reaction (HER).5-6, 48-50 We synthesized MoS2 nanoflowers on graphite electrodes using CVD,5 then measured the electrocatalytic performance of the same electrode before and after ligand functionalization. Figure 5a compares the polarization curves for MoS2 measured before (black) and after (red) 4-fluorobenzyl mercaptan functionalization. A dramatic decrease in catalytic activity was observed for functionalized MoS2 nanoflower sample,


Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with the voltage required to reach a catalytic current density of 10 mA/cm2 increased significantly from 0.33 V to 0.44 V vs. reversible hydrogen electrode (RHE). The decreased HER catalytic activity is further illustrated by comparing the Tafel slopes, a parameter that provides insights into the intrinsic activity and catalytic mechanisms of the catalyst. The Tafel slope increased from 136 mV/decade (black curve) before functionalization to 187 mV/decade (red curve) after functionalization (Figure 5b). Experimental and computational studies have shown that both the edge sites and the sulfur vacancies on MoS2 basal plane contribute to HER catalytic activity.22, 44, 51

Therefore, the decrease in HER performance serves as additional evidence for thiol conjugation

on the edge sites and/or sulfur vacancies on MoS2 basal plane, which results in a significant reduction in available active sites for HER catalysis. This example demonstrates the ability to tune the catalytic properties of MoS2 by controlling its surface chemistry and offers more possibilities for effectively developing MoS2 for various catalytic applications.

Figure 5. Electrochemical characterization of MoS2 nanoflowers for HER electrocatalysis before and after functionalization. (a) Polarization curves and (b) Tafel analysis of the same MoS2 sample measured before (black) and after (red) 4-fluorobenzyl mercaptan functionalization. We further show that ligand functionalization with different thiol-containing bi-functional linkers can also provide new opportunities for introducing different functional groups onto the basal plane of MoS2 MLs and enable a wider range of applications. In particular, MoS2 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

functionalization provides a promising, facile approach for preparing molecularly linked MoS2based heterostructures that can extend the applicability of MoS2 MLs. Utilizing a dithiol organic molecule, we demonstrate the creation of MoS2-quantum dots (QDs) heterostructures. QDs are a class of zero-dimensional, quantum-confined materials with intriguing optical and physical properties.52-53 The QD band gap and properties can be effectively controlled by changing the size of QDs. QD heterostructures can be both promising solar energy conversion materials and interesting model systems for fundamental charge transfer investigation. MoS2-QD heterostructures are particularly appealing because they couple 2D and 0D materials that may lead to unique optical and physical properties, and new applications in optoelectronic devices. Here, we chose 1,5-pentanedithiol as the ligand to functionalize MoS2 surface and link to PbSe QDs. This organic molecule has two thiol groups, one conjugates to MoS2 surface, while the other binds to QDs due to the high affinity of QDs toward thiol moieties.52, 54 The 4 nm diameter PbSe QDs were synthesized using a hot-injection approach.55 We immersed 1,5-pentanedithol functionalized mechanically exfoliated single- and multilayer MoS2 sample on silicon substrates in a suspension of PbSe QDs dispersed in toluene. After 48 h, the MoS2 substrate was washed off in toluene, ethanol, acetone and isopropanol to remove physisorbed QDs. Transmission electron microscopy (TEM) was then used to examine the MoS2 surface. Selected-area electron diffraction (Figure 6a inset) confirmed the observed surface is the basal plane of single crystal 2H-MoS2. As shown in Figure 6a, b, the entire MoS2 basal plane has been completely covered with PbSe QDs. Note the MoS2 sheets in Figure 6b were rolled up at the edge. From that edge, a high-resolution TEM image containing both lattice resolved MoS2 and PbSe QDs can be acquired and the corresponding FFT pattern is shown in Figure 6b inset. The lattice spacings are determined to be 0.62 nm and 0.31 nm that match well with the MoS2 (002) and PbSe (002) spacings, respectively. 14 ACS Paragon Plus Environment

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy-dispersive X-ray spectroscopy (EDS) also revealed the presence of Mo, S, Pb and Se elements (Figure 6c), further confirming the MoS2-PbSe heterostructures. To prove PbSe QDs are indeed linked to MoS2 surface through 1,5-pentanedithiol rather than physisorbed, we conducted FTIR on the MoS2-PbSe heterostructures, and clearly observed peaks that correspond very well with the peaks from the 1,5-pentanedithol molecule (Figure 6d). A control experiment that attached PbSe QDs onto a non-functionalized MoS2 surface showed a much lower QD coverage, confirming that the functionalization plays a key role in creating MoS2-PbSe QD heterostructures. The density of the QDs on MoS2 surface could be limited by the availability of the S vacancies, however, if the density of S vacancies is very high, the QDs could be nearly closed packed with the spacing between QDs mostly determined by repulsion of the surface ligands on the QDs. Such basal plane functionalization chemistry can be utilized to create new MoS2-based heterostructures with QDs,56 other nanomaterials57-58, beyond the typical MX2 materials. Moreover, the high coverage of QDs everywhere on the MoS2 sheets further demonstrates that the thiol-containing molecules are conjugating not only to the edges but also to the basal plane of MoS2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 6. (a, b) TEM images of MoS2-PbSe QD heterostructures at different magnifications with selected-area electron diffraction shown in Figure 6a inset. A fast Fourier transform (FFT)-filtered high-resolution TEM image and its corresponding FFT pattern are shown in Figure 6b inset. (c) EDS spectrum of the MoS2-PbSe QD heterostructures. (d) FTIR spectra of 1,5-pentanedithiol linked MoS2-PbSe QD heterostructure (black) in comparison with free 1,5-pentanedithiol ligand (red).

3. CONCLUSIONS In conclusion, we clearly demonstrate a facile and effective approach to functionalize the basal plane of 2H-MoS2 using thiol conjugation to the numerous S vacancies commonly found in various MoS2 samples. Thorough characterization using FTIR and XPS has confirmed the successful functionalization and revealed various changes in MoS2 chemical environments. We confirmed that S vacancies in MoS2 play the key role in thiol conjugation on the basal plane of MoS2 layers, and controlling S vacancies on MoS2 basal plane can effectively tune the degree of thiol conjugation. Enhanced photoluminescence response and decreased catalytic activity toward HER were also observed and attributed to the passivation of sulfur vacancies, providing additional evidences to the functionalization. Moreover, the successful creation of heterostructures of PbSe QDs on functionalized MoS2 layers with a high surface coverage using dithiol linker molecules serves as a proof-of-concept for taking advantage of the basal plane functionalization to create new MoS2-based heterostructures, further demonstrating the ligand functionalization of MoS2 on the basal plane. This work provides new insights into the surface chemistry of MoS2 and 2D TMDC materials, lays the foundation for further studies of MoS2 surface chemistry on the basal plane, and 16 ACS Paragon Plus Environment

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


facilitates the creation of MoS2 monolayers with well controlled properties for different applications.

4. EXPERIMENTAL 4.1 Synthesis of MoS2 monolayers via chemical vapor deposition (CVD). MoS2 monolayers were synthesized via a MoO3 film sulfidation reaction similar to methods previously reported.41 The synthesis was conducted in a home-built CVD system that consists of a quartz tube (1 inch O.D.) connected to a vacuum pump with pressure and gas flow controllers, and a tube furnace (Thermal Fisher, Lindberg Blue M). In a typical reaction, 1 nm of Mo metal (Kurt J. Lesker, 99.95%) was deposited via electron-beam evaporation onto a Si/SiO2 or sapphire substrates at a rate of 0.05 Å/s. The Mo thin film was oxidized for 60 minutes at 400 ˚C under a pressure of 300 Torr with flow rates of 25 sccm Ar and O2. Then the flow of O2 was discontinued, the pressure was increased to 760 Torr and the temperature of the MoO3 substrates increased to 750 ˚C. Upon reaching 750 ˚C, 200 mg of sulfur placed in an alumina boat was pushed to 18 cm upstream from the center of the furnace, and the reaction was allowed to proceed for 10 minutes, before the furnace was subsequently cooled down naturally to room temperature. Then MoS2 samples were collected and used without any further preparation. All chemicals and reagents were purchased from Sigma-Aldrich and used as received unless noted otherwise. 4.2 Preparation of single- and multilayer MoS2 via mechanical exfoliation. Single- and multilayer MoS2 films were isolated from bulk MoS2 crystals (429MS-AB, small molybdenum disulfide crystal, SPI Supplies) using the scotch tape-based mechanical exfoliation method and then transferred onto the freshly cleaned Si substrates covered by a 100 nm-thick SiO2 layer.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

4.3 Synthesis of MoS2 nanoflowers on graphite substrates. The CVD synthesis of MoS2 nanoflowers followed a reported procedure.5 In a typical reaction, 50 mg of molybdenum (V) chloride (MoCl5, 95%) and 200 mg elemental sulfur (99.5%) were put in alumina combustion boats and used as reaction precursors. The pressure was set to 770 Torr under a steady flow of Ar gas at 125 sccm, and the tube furnace was heated to 525 ˚C at 780 Torr for 20 min to grow MoS 2 nanoflowers directly on graphite substrates. 4.4 Functionalization of MoS2 monolayers. The CVD MoS2 ML sample on Si/SiO2 substrate (or the mechanically exfoliated MoS2 or CVD grown MoS2 nanoflower samples) was soaked in neat 4-fluorobenzyl mercaptan liquid (or neat 1,5-pentanedithol liquid or 5-(trifluoromethyl) pyridine2-thiol aqueous suspension) for 48 h. Afterwards, the substrate was soaked in ethanol, acetone, isopropanol and water for 10 min each, and rinsed harshly with these solvents to wash off physisorbed molecules. 4.5 Preparation of MoS2-PbSe quantum dot heterostructures. PbSe quantum dots were synthesized via a hot-injection approach modified from an established procedure.55 Mechanically exfoliated single- and multilayer MoS2 sample was first functionalized with 1,5-pentanedithiol. Then the functionalized MoS2 sample was soaked in a suspension of PbSe quantum dots (QDs) in toluene. After 48 h, the MoS2 sample was washed vigorously with toluene, ethanol, acetone and isopropanol to remove physisorbed PbSe QDs. 4.6 Structural characterizations. The optical images of all nanostructures were obtained on an Olympus BX51M optical microscope. Fourier transform infrared spectroscopy (FTIR) was conducted on Bruker Tensor 27 system equipped with a room temperature DTGS detector, midIR source (4000 to 400 cm-1), and a KBr beamsplitter. High-resolution X-ray photoelectron


Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectroscopy (XPS) measurements were taken using a Thermo Al Kα XPS with a 180° double focusing hemispherical analyzer and 128-channel detector (60° angular acceptance). All X-ray photoelectron spectra are shifted so that the adventitious carbon C1s peak is at 284.8 eV to make sure the data across samples are aligned and that the samples are not experiencing differential charging effects that may complicate the data. Photoluminescence (PL) spectra were taken using a Thermo Scientific DXR confocal Raman microscope using a 532 nm excitation laser. The sample for TEM analysis was prepared by dry transfer of MoS2-PbSe QD heterostructures onto a TEM grid (Ted Pella, lacey carbon type-A support film, 300 mesh, copper, no. 01890F). The TEM images were acquired on a FEI Titan scanning TEM at an accelerating voltage of 200 kV. Energydispersive X-ray spectroscopy (EDS) was performed using the same TEM equipped with an EDX detector. 4.7 Electron paramagnetic resonance spectroscopy measurement (EPR). Electron paramagnetic resonance measurements were performed on a Bruker ELEXSYS E500 EPR Spectrometer at room temperature. The MoS2 monolayers on a Si/SiO2 substrate were loaded in a quartz tube and measured at a microwave power of 2 mW. 4.8 Electrochemical characterizations. All electrochemical measurements were performed in a three-electrode configuration using a rotating disk electrode (RDE) system (Bioanalytical Systems, Inc.; RDE-2) with a Bio-Logic SP-200 potentiostat in an electrolyte of 0.5 M H2SO4 (aq), which was continuously purged with H2(g) (99.999%). A saturated calomel reference electrode (SCE) (CH Instruments) was used as the reference electrode and a graphite rod (National Carbon Co., AGKSP Spectroscopic Electrode) was used as the counter electrode. Silver paint (Ted Pella, PELCO Colloidal Silver) was used to affix the graphite disks covered with MoS2 catalysts to a RDE glassy carbon tip. The SCE was calibrated against the reversible hydrogen potential (RHE) 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

using a platinum wire (Kurt J. Lesker, 99.99%; 0.50 mm diameter) as the working and counter electrodes to allow potentials to be referenced against a RHE. Cyclic voltammograms were measured from +0.15 V to −0.45 V vs. RHE at a scan rate of 3 mV s−1 at a rotating speed of 2000 RPM. The electrochemical impedance spectroscopy (EIS) was performed at −0.32 V vs RHE, applying a sinusoidal voltage with an amplitude of 10 mV and scanning frequency from 200 kHz to 50 mHz. The EIS spectra were modeled using a simplified Randles equivalent circuit to extract the series resistance (R), which was then subtracted from the polarization curves to correct for iR losses.

SUPPORTING INFORMATION Detailed information about experiment methods. Additional XPS and FTIR figures for functionalization of other ligand molecules on different MoS2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS This work is supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award DE-FG02-09ER46664. This work made use of the EPR instrument that has been supported by the National Science Foundation under Grant No. NSF CHE-0741901.


Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The Chemistry of TwoDimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263-275. (3) Xu, M. S.; Liang, T.; Shi, M. M.; Chen, H. Z., Graphene-Like Two-Dimensional Materials. Chem. Rev. 2013, 113, 3766-3798. (4) Butler, S. Z.; Hollen, S. M.; Cao, L. Y.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J. X.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E., Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926. (5) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L. S.; Jin, S., Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274-10277. (6) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F., Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials. ACS Catal. 2014, 4, 3957-3971. (7) Mattheis, L. F., Band Structures of Transition-Metal-Dichalcogenide Layer Compounds. Phys. Rev. B 1973, 8, 3719-3740. (8) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS2: A New DirectGap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

(9) Mouri, S.; Miyauchi, Y.; Matsuda, K., Tunable Photoluminescence of Monolayer MoS2 Via Chemical Doping. Nano Lett. 2013, 13, 5944-5948. (10) Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H., Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74-80. (11) Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S., MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695-3700. (12) Wu, S. F.; Buckley, S.; Schaibley, J. R.; Feng, L. F.; Yan, J. Q.; Mandrus, D. G.; Hatami, F.; Yao, W.; Vuckovic, J.; Majumdar, A.; Xu, X. D., Monolayer Semiconductor Nanocavity Lasers with Ultralow Thresholds. Nature 2015, 520, 69-72. (13) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147-150. (14) Presolski, S.; Pumera, M., Covalent Functionalization of MoS2. Materials Today 2016, 19, 140–145. (15) Chen, X.; McDonald, A. R., Functionalization of Two-Dimensional Transition-Metal Dichalcogenides. Adv. Mater. 2016, 28, 5738-46. (16) Ataca, C.; Ciraci, S., Functionalization of Single-Layer MoS2 Honeycomb Structures. J. Phys. Chem. C 2011, 115, 13303-13311. (17) Tang, Q.; Jiang, D. E., Stabilization and Band-Gap Tuning of the 1T-MoS2 Monolayer by Covalent Functionalization. Chem. Mater. 2015, 27, 3743-3748. (18) Sim, D. M.; Kim, M.; Yim, S.; Choi, M. J.; Choi, J.; Yoo, S.; Jung, Y. S., Controlled Doping of Vacancy-Containing Few-Layer MoS2 Via Highly Stable Thiol-Based Molecular Chemisorption. ACS Nano 2015, 9, 12115-12123.


Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Cho, K.; Min, M.; Kim, T.-Y.; Jeong, H.; Pak, J.; Kim, J.-K.; Jang, J.; Yun, S. J.; Lee, Y. H.; Hong, W.-K., Electrical and Optical Characterization of MoS2 with Sulfur Vacancy Passivation by Treatment with Alkanethiol Molecules. ACS Nano 2015, 9, 8044-8053. (20) Chou, S. S.; De, M.; Kim, J.; Byun, S.; Dykstra, C.; Yu, J.; Huang, J. X.; Dravid, V. P., Ligand Conjugation of Chemically Exfoliated MoS2. J. Am. Chem. Soc. 2013, 135, 4584-4587. (21) Liu, T.; Wang, C.; Gu, X.; Gong, H.; Cheng, L.; Shi, X. Z.; Feng, L. Z.; Sun, B. Q.; Liu, Z., Drug Delivery with Pegylated MoS2 Nano-Sheets for Combined Photothermal and Chemotherapy of Cancer. Adv. Mater. 2014, 26, 3433-3440. (22) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I., Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. (23) Canfield, D.; Parkinson, B. A., Improvement of Energy-Conversion Efficiency by Specific Chemical Treatments of Normal-MoSe2 and Normal-WSe2 Photoanodes. J. Am. Chem. Soc. 1981, 103, 1279-1281. (24) Wang, T. Y.; Zhu, R. Z.; Zhuo, J. Q.; Zhu, Z. W.; Shao, Y. H.; Li, M. X., Direct Detection of DNA below ppb Level Based on Thionin-Functionalized Layered MoS2 Electrochemical Sensors. Anal. Chem. 2014, 86, 12064-12069. (25) Goncalves, R. H.; Fiel, R.; Soares, M. R. S.; Schreiner, W. H.; Silva, C. M. P.; Leite, E. R., Single-Step Exfoliation and Covalent Functionalization of MoS2 Nanosheets by an Organosulfur Reaction. Chem.-Eur. J. 2015, 21, 15583-15588. (26) Kim, J. S.; Yoo, H. W.; Choi, H. O.; Jung, H. T., Tunable Volatile Organic Compounds Sensor by Using Thiolated Ligand Conjugation on MoS2. Nano Lett. 2014, 14, 5941-5947.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

(27) Backes, C.; Berner, N. C.; Chen, X.; Lafargue, P.; LaPlace, P.; Freeley, M.; Duesberg, G. S.; Coleman, J. N.; McDonald, A. R., Functionalization of Liquid-Exfoliated Two-Dimensional 2HMoS2. Angew. Chem. Int. Ed. 2015, 54, 2638-2642. (28) Nguyen, E. P.; Carey, B. J.; Ou, J. Z.; van Embden, J.; Della Gaspera, E.; Chrimes, A. F.; Spencer, M. J. S.; Zhuiykov, S.; Kalantar-zadeh, K.; Daeneke, T., Electronic Tuning of 2D MoS2 through Surface Functionalization. Adv. Mater. 2015, 27, 6225-6229. (29) Voiry, D.; Goswami, A.; Kappera, R.; Silva, C. D. C. E.; Kaplan, D.; Fujita, T.; Chen, M. W.; Asefa, T.; Chhowalla, M., Covalent Functionalization of Monolayered Transition Metal Dichalcogenides by Phase Engineering. Nat. Chem. 2015, 7, 45-49. (30) Knirsch, K. C.; Berner, N. C.; Nerl, H. C.; Cucinotta, C. S.; Gholamvand, Z.; McEvoy, N.; Wang, Z. X.; Abramovic, I.; Vecera, P.; Halik, M.; Sanvito, S.; Duesberg, G. S.; Nicolosi, V.; Hauke, F.; Hirsch, A.; Colernan, J. N.; Backes, C., Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts. ACS Nano 2015, 9, 6018-6030. (31) Zhou, L.; He, B. Z.; Yang, Y.; He, Y. G., Facile Approach to Surface Functionalized MoS2 Nanosheets. RSC Adv. 2014, 4, 32570-32578. (32) Raybaud, P.; Hafner, J.; Kresse, G.; Toulhoat, H., Adsorption of Thiophene on the Catalytically Active Surface of MoS2: An Ab Initio Local-Density-Functional Study. Phys. Rev. Lett. 1998, 80, 1481-1484. (33) Cheng, Z. H.; He, B. Z.; Zhou, L., A General One-Step Approach for in Situ Decoration of MoS2 Nanosheets with Inorganic Nanoparticles. J. Mater. Chem. A 2015, 3, 1042-1048. (34) Jiang, S. D.; Tang, G.; Bai, Z. M.; Wang, Y. Y.; Hu, Y.; Song, L., Surface Functionalization of MoS2 with Poss for Enhancing Thermal, Flame-Retardant and Mechanical Properties in Pva Composites. RSC Adv. 2014, 4, 3253-3262.


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Hong, J. H.; Hu, Z. X.; Probert, M.; Li, K.; Lv, D. H.; Yang, X. N.; Gu, L.; Mao, N. N.; Feng, Q. L.; Xie, L. M.; Zhang, J.; Wu, D. Z.; Zhang, Z. Y.; Jin, C. H.; Ji, W.; Zhang, X. X.; Yuan, J.; Zhang, Z., Exploring Atomic Defects in Molybdenum Disulphide Monolayers. Nat. Commun. 2015, 6, 6293. (36) Qiu, H.; Xu, T.; Wang, Z. L.; Ren, W.; Nan, H. Y.; Ni, Z. H.; Chen, Q.; Yuan, S. J.; Miao, F.; Song, F. Q.; Long, G.; Shi, Y.; Sun, L. T.; Wang, J. L.; Wang, X. R., Hopping Transport through Defect-Induced Localized States in Molybdenum Disulphide. Nat. Commun. 2013, 4, 2642. (37) Zhou, W.; Zou, X. L.; Najmaei, S.; Liu, Z.; Shi, Y. M.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J. C., Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615-2622. (38) Le, D.; Rawal, T. B.; Rahman, T. S., Single-Layer MoS2 with Sulfur Vacancies: Structure and Catalytic Application. J. Phys. Chem. C 2014, 118, 5346-5351. (39) Yu, Z. H.; Pan, Y. M.; Shen, Y. T.; Wang, Z. L.; Ong, Z. Y.; Xu, T.; Xin, R.; Pan, L. J.; Wang, B. G.; Sun, L. T.; Wang, J. L.; Zhang, G.; Zhang, Y. W.; Shi, Y.; Wang, X. R., Towards Intrinsic Charge Transport in Monolayer Molybdenum Disulfide by Defect and Interface Engineering. Nat. Commun. 2014, 5, 5290. (40) Komsa, H. P.; Kurasch, S.; Lehtinen, O.; Kaiser, U.; Krasheninnikov, A. V., From Point to Extended Defects in Two-Dimensional MoS2: Evolution of Atomic Structure under Electron Irradiation. Phys. Rev. B 2013, 88, 035301. (41) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X. L.; Shi, G.; Lei, S. D.; Yakobson, B. I.; Idrobo, J. C.; Ajayan, P. M.; Lou, J., Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754-759.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

(42) Castner, D. G.; Hinds, K.; Grainger, D. W., X-Ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12, 5083-5086. (43) Hsu, T. M.; Lee, J. S.; Hwang, H. L., Photoreflectance of Sulfur-Annealed Copper Indium Disulfide. J. Appl. Phys. 1990, 68, 283-287. (44) Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S., Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 2016, 138, 7965-7972. (45) Cai, L.; He, J. F.; Liu, Q. H.; Yao, T.; Chen, L.; Yan, W. S.; Hu, F. C.; Jiang, Y.; Zhao, Y. D.; Hu, T. D.; Sun, Z. H.; Wei, S. Q., Vacancy-Induced Ferromagnetism of MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 2622-2627. (46) Han, H. V.; Lu, A. Y.; Lu, L. S.; Huang, J. K.; Li, H. N.; Hsu, C. L.; Lin, Y. C.; Chiu, M. H.; Suenaga, K.; Chu, C. W.; Kuo, H. C.; Chang, W. H.; Li, L. J.; Shi, Y. M., Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment. ACS Nano 2016, 10, 1454-1461. (47) Amani, M.; Lien, D. H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; Santosh, K. C.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S. C.; He, J. H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A., Near-Unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350, 1065-1068. (48) Ding, Q.; Meng, F.; English, C. R.; Caban-Acevedo, M.; Shearer, M. J.; Liang, D.; Daniel, A. S.; Hamers, R. J.; Jin, S., Efficient Photoelectrochemical Hydrogen Generation Using


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Heterostructures of Si and Chemically Exfoliated Metallic MoS2. J. Am. Chem. Soc. 2014, 136, 8504-8507. (49) Ding, Q.; Zhai, J. Y.; Caban-Acevedo, M.; Shearer, M. J.; Li, L. S.; Chang, H. C.; Tsai, M. L.; Ma, D. W.; Zhang, X. W.; Hamers, R. J.; He, H.; Jin, S., Designing Efficient Solar-Driven Hydrogen Evolution Photocathodes Using Semitransparent Moq(X)Cl(Y) (Q = S, Se) Catalysts on Si Micropyramids. Adv. Mater. 2015, 27, 6511-6518. (50) Ding, Q.; Song, B.; Xu, P.; Jin, S., Efficient Electrocatalytic and Photoelectrochemical Hydrogen Generation Using MoS2 and Related Compounds. Chem 2016, 1, 699-726. (51) Li, H.; Tsai, C.; Koh, A. L.; Cai, L. L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J. H.; Han, H. S.; Manoharan, H. C.; Abild-Pedersen, F.; Norskov, J. K.; Zheng, X. L., Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2016, 15, 48–53. (52) Selinsky, R. S.; Ding, Q.; Faber, M. S.; Wright, J. C.; Jin, S., Quantum Dot Nanoscale Heterostructures for Solar Energy Conversion. Chem. Soc. Rev. 2013, 42, 2963-2985. (53) Watson, D. F., Linker-Assisted Assembly and Interfacial Electron-Transfer Reactivity of Quantum Dot-Substrate Architectures. J. Phys. Chem. Lett. 2010, 1, 2299-2309. (54) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007-4015. (55) Wehrenberg, B. L.; Wang, C. J.; Guyot-Sionnest, P., Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots. J. Phys. Chem. B 2002, 106, 10634-10640.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

(56) Schornbaum, J.; Winter, B.; Schießl, S. P.; Gannott, F.; Katsukis, G.; Guldi, D. M.; Spiecker, E.; Zaumseil, J., Epitaxial Growth of Pbse Quantum Dots on MoS2 Nanosheets and Their nearInfrared Photoresponse Adv. Funct. Mater. 2014, 24, 5798. (57) Samad, L.; Bladow, S. M.; Ding, Q.; Zhuo, J.; Jacobberger, R. M.; Arnold, M. S.; Jin, S., Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures Via Van Der Waals Epitaxy. ACS Nano 2016, 10, 7039-7046. (58) Jariwala, D.; Marks, T. J.; Hersam, M. C., Mixed-Dimensional Van Der Waals Heterostructures. Nat. Mater. 2016, 16, 170-181.

TOC Graphic


Basal-Plane Ligand Functionalization on ... - ACS Publications

Recommend Documents