Exfoliation Of A Large-area Transition Metal Chalcogenide Monolayer

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Scientific Reports Volume 5, Article Number: 14714 (2015) Cite this article

Isolating large areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. Mechanical peeling technology can provide single layers of the highest structural quality, so that their original characteristics and final device performance can be studied. However, one of the main disadvantages of this technology is the low yield and the small lateral dimensions of the monolayer produced (usually <10 μm). Here, we report a new mechanical peeling technique based on chemically enhanced adhesion that produces MoS2 monolayers with typical lateral dimensions of hundreds of microns. The idea is to take advantage of the chemical affinity of sulfur atoms, and the bonding force of sulfur atoms to the gold surface is stronger than that of adjacent layers of massive molybdenum disulfide crystals. In addition, we found that our peeling process is not specific to MoS2, but can be generally applied to various layered chalcogenides, including selenite and telluride, providing a large area for the entire layered transition metal chalcogenide Easy to obtain two-dimensional crystals.

Layered transition metal chalcogenides (TMC) exhibit strong intra-layer metal-chalcogenide bonds and weak interlayer bonds between adjacent chalcogenide atomic planes. The recent interest in studying their two-dimensional (2D) forms (consisting of three to five layers of atomic sheets) is driven by the fact that the properties of atomically thin crystals may be very different from their well-characterized bulk counterparts. A convincing example is the transition of MoS2 energy band structure from indirect band gap to direct band gap, because the number of layers is reduced from bulk to single layer2, which opens the way for optoelectronic applications3. In addition, the transition metal chalcogenide family is very large, covering a wide range of characteristics from semiconductors (MoS2, WSe2) to semi-metals (TiS2, TiSe2), from topological insulators (Bi2Te3, Bi2Se3) to related materials (NbS2, NbSe2). Such a wide range of properties have great potential in basic research and applications, even if graphene has unique versatility, it cannot compete with it.

The easy access to large TMC monolayers is critical to exploring their properties, which is similar to the way in which the easy separation of large, high-quality graphene flakes has led to significant progress in graphene research. Since several large TMC crystals are layered materials, similar to graphite, a single TMC layer can be separated by mechanical peeling4. However, by using the most widely used micromechanical cleavage ("scotch tape") technology, the lateral dimensions of the peeled TMC monolayer are usually on the order of 2, 3, 4, and 5 microns, which is about an order of magnitude smaller than that which can be conventionally achieved by the same technology. The lateral dimensions of the graphene flakes obtained. The reason for this situation may be due to the unique mechanical strength6 and super adhesion7 of graphene to SiO2, which is unmatched by the outstanding mechanical properties of TMCs 8 in other aspects.

Alternative methods for the production of monolayers of various TMC materials are chemical9,10 and liquid phase stripping of the monolayer from its bulk crystals11, and chemical vapor deposition (CVD) growth of the TMC layer12,13. In principle, CVD technology can grow the macroscopic area of TMC film; however, it is still difficult to continuously grow a single layer on the macroscopic scale. In addition, the CVD growth of various TMC materials requires the development and optimization of dedicated experimental devices for the growth of each TMC material. Chemically peeled TMC layers usually undergo irreversible chemical modification of their structure and properties, and their lateral dimensions are usually less than 1 micron9. Recently, electrochemical exfoliation of MoS2 flakes with lateral dimensions up to 50 microns has been demonstrated; however, even these layers can undergo partial oxidation10. Ultrasound-assisted liquid phase exfoliation produces a large number of such exfoliated TMC layers in various solvents, but most of the flakes are multi-layered, and their lateral dimensions are usually only on the order of a few hundred nanometers.

Compared with other methods, mechanical exfoliation provides a 2D TMC sheet with high structural quality, which can conduct basic research on its original characteristics and final device performance, similar to graphene. Due to its superior structure, most of the basic findings have been exfoliated samples Upper realization and electronic quality 14,15. The main limitation of the micromechanical peeling of various TMC materials is the small single-layer yield and the relatively small lateral size, usually only a few microns, which makes subsequent research and device manufacturing more difficult.

We have developed a novel mechanical peeling technology that overcomes the limitations of transparent tape technology and can peel off TMC monolayers with lateral dimensions in the range of hundreds of microns. In the mechanical peeling process, the isolation of the single layer is possible because the adhesion of the bottom layer to the substrate becomes stronger than the adhesion of the bulk crystal itself. Graphene shows super strong adhesion to SiO27, which can play an important role in the peeling process and promote the peeling of large (usually tens of microns) single layer. In order to improve the peeling process of MoS2 monolayers, we recommend improving their adhesion to the substrate. In order to achieve this, we use a gold substrate instead of the commonly used SiO2/Si, and use the chemical affinity of sulfur to gold to achieve stronger adhesion. A gold substrate with a large, atomically flat and clean Au (111) surface was prepared by epitaxially growing a gold film about 100 nm thick on mica. These gold films were freshly lysed from the mica surface before the experiment. Next, a thermal peeling tape was used to peel the thick multilayer sheet from the massive molybdenum disulfide crystals. These flakes have been placed on top of the newly cut gold substrate. By heating the sample to 90 °C, the thick molybdenum disulfide flakes have been released from the tape onto the gold substrate. We use a short time ultrasonic treatment (70 W, 1-10 seconds) in acetone to remove thick molybdenum disulfide flakes from the gold surface. We found that after a few seconds of sonication, a few thick slices had fallen off; however, under them, the last (bottom) MoS2 layer was still attached to the gold substrate.

In the optical microscope image, the MoS2 single layer can be identified as the area with the weakest color contrast, as shown in Figure 1. The optical image shows that a large area of hundreds of microns is covered by a thin MoS2 layer. In order to confirm that these areas with the weakest optical contrast are indeed single-layer MoS2, we performed a confocal Raman spectroscopy measurement (λ = 532 nm, 1 mW), as shown in the inset of Figure 1b. We have observed that the characteristic MoS2 peak 18 (E2g, A1g) is near 384 and 404 cm-1 wave numbers, respectively. This indicates that the large area of hundreds of microns observed by the optical microscope can be identified as a MoS2 monolayer 4,18,19. The large flakes peeled off on top of the conductive Au substrate allow us to perform scanning tunneling microscope (STM) measurements on the mechanically peeled single-layer MoS2 flakes. From the STM measurement, it is found that the height of the studied layer relative to the Au substrate is about 0.7 /- 0.1 nm (Figure 2a), which confirms the single layer thickness of the peeled flakes. Even under environmental conditions, atomic resolution images can be routinely obtained (Figure 2b). A hexagonal atomic lattice with a periodicity of 3.1 /- 0.1 Å has been revealed, which corresponds to the lattice constant 20 of the top sulfur atom.

Mechanically peeled off by chemically enhancing adhesion of a large area of molybdenum disulfide monolayer.

Optical microscope images (a, b) of a MoS2 monolayer area (1L, outlined by a dotted line) peeled off on a gold (Au 111) substrate with a lateral size of several hundred microns. The blue flakes are thick molybdenum disulfide multilayer films (ML). Raman spectroscopy (inset) has confirmed that the large area with the weakest optical contrast is a single layer.

Atomic structure of MoS2 monolayer.

(a) STM image (500 mV, 1 nA) of MoS2 single layer peeled off to gold substrate. It was found that the relative height of the MoS2 layer was 0.7 /- 0.1 nm, proving its monolayer nature. The line cut shown is taken along the direction where the green line passes through the edge markings. The darker areas of tens of nanometers are atomic-scale pits on the gold surface. (b) Atomic resolution STM image (5 mV, 2 nA) of the MoS2 monolayer, showing that the hexagonal lattice has a periodicity of 0.31/-0.01 nm (see inset), corresponding to the lattice of sulfur atoms on the top.

Although gold substrates are ideal for STM measurements, several experimental techniques, including electrical transmission measurement and device applications, require insulating substrates. In addition, it is important to understand the bonding properties between the MoS2 monolayer and the gold substrate, and the most important thing is whether it is completely reversible. Raman spectroscopy and atomic resolution STM measurements show no significant deviation from the internal structure of MoS2. However, in order to completely clarify this issue, we have transferred the exfoliated MoS2 monolayer from gold to the SiO2/Si substrate for further study. To achieve this, a piece of heat release tape was placed on top of the peeled MoS2/Au sample. Then, the gold substrate was etched away using potassium iodide solution. The molybdenum disulfide sheet supported by the thermal release tape can then be transferred to any desired substrate. After heating the sample, the flakes will be released on the surface of the new substrate; in our case it is SiO2/Si. Figure 3a shows an atomic force microscope image of MoS2 flakes transferred on top of SiO2/Si. The Raman spectrum of the MoS2 flakes transferred from gold to SiO2/Si (Figure 3b) perfectly matches the characteristic spectrum of the MoS2 monolayer directly peeled off to the SiO2/Si substrate 18,19, indicating that the MoS2 layer can be reversibly separated from the gold surface After exfoliating, there is no obvious structural change.

MoS2 single layer on insulating substrate.

(a) AFM image of MoS2 layer peeled off into gold and transferred to SiO2/Si substrate. (b) The Raman spectrum of the transferred monolayer (λ = 532 nm, 1 mW) proves that the MoS2 layer can be reversibly separated from the Au substrate without changing its structure.

We also investigated whether the exfoliation process is specific to molybdenum disulfide or can be applied more generally to various layered materials. We found that our stripping method produces large areas of molybdenum disulfide flakes that are not specific to molybdenum disulfide or even sulfides, but are equally effective for various layered chalcogenides (including selenides and tellurides). To illustrate this in Figure 4, we show an optical microscope image of a WSe2 and Bi2Te3 monolayer with a lateral scale of hundreds of microns. However, when we tried to apply this technique to exfoliate graphene, we found that after a short ultrasonic treatment, all the thick graphite flakes had been removed from the gold substrate, leaving no graphene layer at all. This further supports that the mechanism behind our peeling process is indeed the chemically enhanced adhesion of various layered TMC materials. In addition, compared with the standard scotch tape method, the peeling technique reported here can in principle achieve high coverage of macroscopic substrates with various TMC monolayers.

Exfoliation of various layered chalcogenide monolayers.

Optical microscope images of WSe2 (a) and Bi2Te3 (b) monolayers. The inset shows the Raman spectrum of WSe2 (a) and the AFM spectrum cut on the Bi2Te3 step edge (b), confirming the single-layer nature of the exfoliated large-area flakes.

In short, we have demonstrated a simple and versatile method to isolate various laminar transition metal chalcogenides in a large area single layer, and use their chemically enhanced adhesion to the gold substrate to explore the new characteristics of two-dimensional crystals for the system. The way was opened up and the entire series of layered chalcogenides was formed.

Optical imaging has been performed by Zeiss Axio Imager microscope. Scanning tunneling microscope measurements have been performed on the Nanooscope E instrument under environmental conditions. Atomic resolution images can be routinely achieved with typical settings of Ub = 5–100 mV bias voltage and It = 1–3 nA tunnel current. Bruker Multimode 8 AFM acquired AFM images in tap mode. The contamination found by AFM on the MoS2 flakes transferred to the SiO2/Si substrate is likely to be the residue of gold nanoparticles from the wet etching process of the gold substrate. Raman spectroscopy has been obtained by a Witec 300RSA confocal Raman microscope using a 532 nm laser wavelength and a typical power of 0.2–2 mW. Raman spectroscopy is also used to confirm the monolayer properties of WSe2 flakes, and the atomically thin Bi2Te3 layer is very unstable even for low (0.2 mW) laser power. We have found that it is important to use freshly cut stripped gold substrates; however, their source and flame annealing are not important for obtaining good results. Commercially available heat release tapes have been used in all transfer experiments.

How to cite this article: Magda, GZ, etc. Exfoliation of a large area of transition metal chalcogenide monolayer. science. Rep. 5, 14714; doi: 10.1038/srep14714 (2015).

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