Selective Adsorption of Thiol Molecules at Sulfur Vacancies on MoS2

Oct 2, 2012 - derivatives dodecanethiol and (3-mercaptopropyl)- trimethoxysilane (MPS) on a MoS2(0001) substrate at prefabricated sulfur vacancies...
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Selective Adsorption of Thiol Molecules at Sulfur Vacancies on MoS2(0001), Followed by Vacancy Repair via S−C Dissociation Marina Makarova,* Yuji Okawa,* and Masakazu Aono International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: For the development of various molecular nanostructures such as single-molecule electronic circuits, it is very important to fix the molecular components at predetermined positions on a substrate. We report the fixation of the thiol derivatives dodecanethiol and (3-mercaptopropyl)trimethoxysilane (MPS) on a MoS 2 (0001) substrate at prefabricated sulfur vacancies. Scanning tunneling microscopy (STM) reveals the selective bonding of thiol groups to the Mo atoms at the vacancy defects. In addition, we report STM tipinduced dissociation of the S−C bond, which essentially results in the repair of the vacancy defects with sulfur atoms from the thiols. This is consistent with the high desulfurization reactivity of MoS2. Because of its structure and composition, MPS has a higher dissociation reactivity than that of dodecanethiol.

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INTRODUCTION In single molecule electronics, each basic electronic operation is performed by a single functional molecule, resulting in miniaturization of the devices and the development of cheaper and more environmentally friendly manufacturing. Since Aviram and Ratner proposed a single-molecular rectifier,1 considerable work has been done,2−10 but the realization of a practical single molecule circuit has yet to be achieved. One of the most difficult problems is fixing each functional molecule to a substrate at designated positions and then interconnecting them. Recently, we developed a novel method of interconnection by initiating chain polymerization of diacetylene compounds with the tip of a scanning tunneling microscope (STM).11−13 This “chemical soldering” enables the covalent connection of a single conductive polydiacetylene chain to single functional molecules.14,15 As for fixing the latter at designated positions on a substrate, manipulation with scanning probe microscope (SPM) tips16−22 can be done, but this method usually requires low temperatures. Another method would be fabrication of active sites on a substrate at which functional molecules selectively adsorb. Examples include selective adsorption of single molecules at dangling bonds fabricated by a STM tip on a hydrogen-terminated Si(100) surface,23−26 adsorption of molecules in monolayer-deep pits produced on an alkali halide surface,27 and replacement of conjugated molecules in an alkanethiol matrix on an Au(111) surface by STM lithography.28 To interconnect fixed single molecules with conductive polymer chains using the chemical soldering method, the (0001) basal faces of graphite11,12 or MoS213,29 are favored as substrates, because diacetylene monomers form self-assembled monolayers on them. MoS2, being a moderate band gap semiconductor (1.2 eV30), is a better substrate than metallic graphite on which to study © 2012 American Chemical Society

electrical properties of molecules. We thus investigated the fixation of organic molecules on a basal plane of MoS2. MoS2 consists of a molybdenum layer sandwiched between two sulfur layers. The flat (0001) basal plane of MoS2 is very inert, so that most molecules are physisorbed, as shown by many STM studies.13,29,31−35 Plane edges and defects in the plane are very chemically reactive, however, and thus MoS2 is well-known as a catalyst for hydrodesulfurization,36−41 which is used for sulfur removal from oil. Sulfur-containing groups have a tendency to form covalent bonds with unsaturated Mo edges or vacancy defects in the sulfur layer.36−40,42 Hydrodesulfurization is based on electron donation from MoS2 to the S−C antibonding orbital, which induces bond weakening and ultimately scission at hydrogenation. The fact that sulfur vacancies on the basal plane act as catalytic reaction sites37,38 suggests that sulfur-containing thiol derivatives will selectively chemisorb there. Furthermore, selective extraction of sulfur atoms in the topmost layer has been demonstrated by the application of a pulsed bias voltage at a STM tip.43,44 It is thus feasible to fix thiols at any predetermined position on the basal plane of MoS2 by first applying STM-tip extraction of sulfur atoms. We examined the adsorption of two thiol derivatives. The first is the simple alkane dodecanethiol (CH3(CH2)11SH), which has been used to stabilize gold nanoparticles.45 The second is the bifunctional short-chain alkoxysilane (3mercaptopropyl)trimethoxysilane (MPS; (CH 3 O) 3 Si(CH2)3SH), which has been used as a molecular adhesive between metal (or semiconductor) and oxide surfaces, because Received: July 23, 2012 Revised: September 26, 2012 Published: October 2, 2012 22411

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Figure 1. (a) STM image of a prepared MoS2(0001) surface (I = 3 nA, Vs = −1 V) containing sulfur vacancies (black areas). (b) Magnified STM image of one of the sulfur vacancies (I = 3 nA, Vs = −1 V). (c−f) Series of STM images of MoS2(0001) surface after adsorption of dodecanethiol molecules. A stable defect marked “Defect” is used as a reference point. (c) STM image just after adsorption (I = 25 pA, Vs = −1 V). (d, e) A part of panel c is scanned in a “brushing mode” (I = 3 nA, Vs = −0.5 V). Bright spots indicated by the arrows in panel d disappear in panel e. All bright spots disappear by scanning at I = 5 nA, Vs = −0.5 V. (f) Image under normal scanning conditions (I = 25 pA, Vs = −1 V). A cleared bright-spot-free area is observed in the dotted square. (g) Atomic image of MoS2(0001) is observed in the cleared area (I = 5 nA, Vs = −0.5 V). (h−k) Schematic of probable processes (see the text).

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EXPERIMENTAL SECTION Substrates were prepared from thin 10−50 mm2 platelets that were exfoliated from a MoS2 single crystal (SPI Supplies), covered by Diemat 2700P low temperature sealing glass paste, and placed between two glass substrates in a sandwich-like structure. These were heated on a hot plate in air at 570 K to remove organic residues and then at 810 K to glaze and bind the MoS2 to each glass surface. The structures were kept for 30 min at this temperature with periodical turning for uniform heating. After cooling, the glass pieces were separated with a razor blade, creating newly cleaved MoS2(0001) surfaces on

its thiol tail forms strong covalent bonds to Ag, Au, Cu, Pt, and GaAs and its methoxy headgroup can undergo hydrolysis and condensation reactions.46−50 Thus, the thiol tail should selectively attach to sulfur vacancies on a MoS2(0001) surface, while other molecules or nanoparticles could be fixed to the methoxy headgroup. Moreover, in the case of a high-density adsorbed MPS film, the polysiloxane formed by hydrolysis of the methoxy headgroup could be used as an insulating layer.47,48 22412

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after removal of the bright spots, and atomic images of MoS2(0001) surface were observed as shown in Figure 1g. The above results indicate a chemisorption and dissociation process schematically shown in Figure 1h−k, which is based on previously reported catalytic reactions.37,38 First, thiol groups of the molecules selectively chemisorb at the sulfur vacancies (Figure 1i), bonding with Mo atoms. Electrons are donated from lone pairs of S to empty 4d orbitals of Mo and backdonated from other 4d Mo orbitals to the S−C antibonding orbital.36 As a result, the S−C bonds are first weakened and then easily dissociated during scanning (Figure 1j). After dissociation, the alkyl parts of the molecules desorb, while the sulfur atoms remain coordinated to the Mo (Figure 1k). For dodecanethiol molecules adsorbed on an Au(111) surface, the Au−S bonds are dissociated by stimulation with a STM tip, so that whole dodecanethiol molecules desorb.28 In contrast, we see that for dodecanethiol molecules adsorbed at sulfur vacancies on the MoS2(0001) surface, the S−C bonds are dissociated by the STM tip, leaving the sulfur atoms on the surface. This result is consistent with the high desulfurization reactivity of defective MoS2. As mentioned above, the defect-free area of a MoS2 basal plane is inert, so only physisorption is expected. We can observe physisorbed dodecanethiol with AFM. Figure 2a depicts an AFM image of dodecanethiol on a MoS2(0001) substrate, which was prepared similarly to the sample shown in Figure 1c. Figure 2b is a height profile along the line in Figure 2a. Because of limited AFM resolution, we could not observe the chemisorbed molecules at defects that had been seen with STM. There are two types of physisorbed structures visible in the AFM images. The first are flat areas with an irregular shape and a height of about 3 nm, as indicated by the label A in Figure 2a,b. The second are deformed spheres of 10−80 nm in height, as indicated by label B. The AFM phase image (inset of Figure 2a) indicates that both have the same phase shift, which suggests that they consist of the same material. Since the chain length of a dodecanethiol molecule is 1.52 nm, the flat areas A could be vertically oriented dodecanethiol bilayers. In contrast, many other alkane, alkanol, alkanoic acid, and alkanamide molecules lie flat on the MoS2(0001) substrate.13,29,33−35 The vertically oriented dodecanethiol is consistent with the report that the interaction of MoS2(0001) with thiol groups is more favored over that with molecules having amino or carboxy groups.42 This is likely because thiol groups have a high affinity for MoS2(0001) by interacting with the protruding Mo dz2 orbital over the S layer. The deformed spheres B are probably self-organized dodecanethiol structures such as micelles, as schematically depicted in Figure 2c. It should be noted that neither the flat nor the spherical arrays of dodecanethiol molecules were observed with STM. This indicates that they are weakly physisorbed and are brushed away by the STM tip even at I = 25 pA. 3. Adsorption and Dissociation of MPS. Figure 3a shows a STM image of a MoS2(0001) substrate after treatment with MPS, where Vs = −1 V and I = 25 pA. As with dodecanethiol, the bright spots are attributed to the adsorption of MPS molecules at the preformed sulfur vacancies. However, these bright spots were much more unstable during low current STM scanning than those due to dodecanethiol adsorption. They disappeared after two or three scans at I = 25 pA and Vs = −1 V (Figure 3b), resulting in an uncovered, vacancy-free MoS2(0001) surface. Thus, the same adsorption−dissociation mechanism occurs as described above (Figure 1h−k); however,

each. These were heated in vacuum for 1 h at 670 K for degassing. After cooling, the substrates were then incubated for 90 h in a fresh solution of 10 mL of chloroform (Wako) and 0.3 mL of dodecanethiol (Kanto Chemical) or MPS (Aldrich). No color changes or solid-phase precipitations were observed in the solution. After removal from the thiol solutions, the samples were washed with ethanol, dried in air, and baked on a hot plate at 390 K for 30 min. STM experiments were performed in air at room temperature using a NanoScope STM system (Bruker) with Pt−Ir tips operated in constant current mode. The sample bias voltage (Vs) and the tunneling current (I) for observation were typically −1 V and 25 pA, respectively. Atomic force microscopy (AFM) experiments were performed in air at room temperature using an Agilent 5500 system. Height, amplitude, and phase images in ac mode were recorded simultaneously. Commercial silicon cantilevers (PPP-NCH, NanoWorld) with nominal spring constants of 33−38 N m1− and resonance frequencies of 280−320 kHz were used.

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RESULTS AND DISCUSSION 1. MoS2 Substrate Characterization. Since a thin platelet of MoS2 is very soft, it is better to fix it on a harder substrate such as glass or silicon. The fixation enables more stable observation of SPM and easier handling. Because epoxies or organic glues get destroyed by high temperatures, as well as by some organic solvents, the novel low-temperature sealing glass method described above allowed us to create completely organic-free samples. STM images of MoS2 platelets attached to glass substrates are shown in Figure 1. The MoS2(0001) prior to thiol adsorption exhibits many dark spots, as shown in Figure 1a; the spot density varies across the substrate. They have been identified as sulfur vacancies on the MoS 2 basal plane,41,43,44,51−53 which can be clearly seen in a magnified STM image (Figure 1b). We next investigated the selective chemisorption of thiol derivatives at the sulfur vacancies. 2. Adsorption and Dissociation of Dodecanethiol. Figure 1c shows a STM image of a MoS2(0001) substrate after treatment with dodecanethiol solution, observed at Vs = −1 V and I = 25 pA. In this image, we see the appearance of many bright spots on the surface and few or no dark spots. Since the density of bright spots in Figure 1c is roughly equivalent to that of the dark spots in Figure 1a, we assume that the bright spots are dodecanethiol molecules selectively adsorbed at the sulfur vacancies. In Figure 1c, there is a very bright large protrusion (labeled “Defect”). Since this protrusion is very stable (as shown below) and it has the characteristic MoS2 atomic lattice in STM images (not shown), it is probably due to a defect or impurity beneath the topmost surface.54,55 We use it as a fiducial reference point in the STM images. The bright (dodecanethiol) spots are stable during scanning at low currents (I = 25 pA) and medium voltage (Vs = −1 V), but they start to disappear at Vs = −0.5 V and I = 3 nA (“brushing mode”). Figure 1d,e shows a series of STM images obtained for a section of Figure 1c; we see that the bright spots indicated by the arrows in Figure 1d are absent in Figure 1e. Finally, the bright spots were removed completely at I = 5 nA. After going from brushing mode to normal imaging mode (Vs = −1 V, I = 25 pA), we can see in Figure 1f the bright-spot-free window (indicated by the dotted square) where brushing mode scanning had been performed near the large protruded defect. Surprisingly, darker spots (sulfur vacancies) were not observed 22413

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Figure 2. (a) AFM image of physisorbed dodecanethiol molecules on MoS2(0001). Labels A and B indicate the two types of physisorbed states. The inset shows a phase contrast image. (b) Height profile along the line in panel a. (c) Schematic of two possible physisorbed states.

Figure 3. SPM images of MPS molecules adsorbed on a MoS2(0001) surface. (a) STM image obtained just after adsorption (I = 25 pA, Vs = −1 V). (b) Second STM image obtained in the same area. Bright spots in panel a disappear in b. (c) AFM image of MoS2(0001) surface after adsorption.

chained mercaptoalkoxysilanes would be more stable on MoS2(0001) than MPS. As with dodecanethiol, the AFM image in Figure 3c indicates physisorbed MPS molecules. Much of the surface is covered with quasispherical objects of 10−20 nm in height. The dark areas are probably uncovered MoS2, which is more clearly displayed by the phase image (inset of Figure 3c). The spherical objects are likely micelles of MPS, similar to those observed for dodecanethiol in Figure 2. In contrast to the case of dodecanethiol, no flat layers of MPS are visible.

the S−C bonding in MPS appears to be weaker than that of dodecanethiol. This could be explained by the inductive effect, whereby the CH3−O− groups act as electron acceptors, causing the MPS molecule to be much more acidic than dodecanethiol; the latter has a long alkyl chain that acts as a donor and enhances base properties at the SH group. The acidic properties should lead to a lower energy and a higher occupancy of the S−C antibonding orbital in MPS, making it weak enough to be broken at low tunneling currents. The effect of molecule structure on S−C bond strength was observed for other alkanethiols at sulfur vacancy defect sites on MoS2(0001), where the S−C bond dissociation temperature increases with chain length from methanethiol (about 300 K)37 to ethanethiol (about 570 K).38 This suggests that the enhancement of donor properties could strengthen the S−C bond and that the long-

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CONCLUSIONS Selective adsorption of the thiol derivatives dodecanethiol and MPS at sulfur vacancies on a MoS2(0001) substrate was demonstrated. As described above, selective extraction of sulfur 22414

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atoms has been demonstrated with a STM tip.43,44 These preformed vacancies could be used as adsorption centers for thiol derivatives. Using bifunctional molecules such as MPS, we would be able to further functionalize the surface with other molecular or nanoparticle elements. We could thus arrange molecular elements at any designated positions on the surface. Furthermore, we would also be able to connect them to single polydiacetylene chains using the chemical soldering method. Overall, these techniques will enable the fabrication of novel molecular nanostructures, including single molecule devices. Dissociation of the S−C bond of thiol derivatives on a MoS2(0001) substrate was also observed, where the reactivity of dodecanethiol was less than that of MPS. This observation is important from a catalytic point of view for hydrodesulfurization reactions on MoS2. This reaction is also useful to repair the sulfur vacancies on the surface with sulfur atoms from the adsorbed molecules. Single-layers or bilayers of MoS 2 have lately attracted considerable attention as materials for use in next-generation nanoelectronic devices, because their large intrinsic band gap gives them an advantage over graphene, which does not have a band gap.56−58 Atomic scale vacancy-defects in thin layers of MoS2, however, significantly alter electronic properties; a large dependence of conductivity on the concentration of sulfur vacancies in bulk MoS2 single crystal was previously reported,59,60 and modification of the local electronic state around sulfur vacancies was recently demonstrated by a scanning tunneling spectroscopy measurement.61 Hence, the method for repairing sulfur vacancies reported here will help to control electronic properties of thin-layer-MoS2 devices, and control of vacancy defects will be very important for device high-performance and reproducible behavior in general. Since the repair of sulfur vacancies is limited to the STM tip scan area, we will be able to alter the local electronic properties. However, STM tip scanning is too slow for repairing a macroscopic area on the surface. Alternatives would be substrate heating37,59 or electron beam irradiation to dissociate the S−C bond of thiol derivatives over a wide area of the MoS2 surface in a short time.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.M.); OKAWA. [email protected] (Y.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the World Premier International Research Center Initiative (WPI), the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and partially supported by JSPS KAKENHI Grant Number 24241047.

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