Top-down synthesis techniques include mechanical (scotch-tape method), liquid/chemical, and intercalation based exfoliation methods to produce high quality and large area TMCs. These techniques have largely been employed for the synthesis of graphene layered structur es and recently being extended for layered TMCs. Here, a target bulk material is exfoliated through various techniques to obtain few-layered structures. These methods have been briefed in the following sections.
Mechanical Exfoliation Technique
The mechanical exfoliation technique is by far the most efficient technique to yield highly crystalline and extremely thin narrosheets of layered TMCs. hr this technique, the bulk TMCs are the fust subject to a mechanical peeling procedure in which loosely bound layers (connected to each other through van der Waals interactions) are gathered by an adhesive scotch tape (Figure 1.9, top panel). The scotch tape with cleaved samples are then nibbed onto the surface of a suitable substrate material, where the tape is repeatedly nibbed to cleave the sample further and upon removal of the tape, it would leave single- and/or multi-layered TMCs on the substrate surface (Figure 1.9, bottom panels a-d). Though the process of using a scotch tape produces high- quality single layers of TMDCs, it cannot be used for scalable synthesis of desired layered stmctures. Therefore, the technological application of scotch tape assisted mechanical exfoliation is rather limited. As a way to exfoliate large-area MLs, it was proposed to make use of chemically enhanced adhesion. The use of chemical affinity of sulfur atoms that can bind to a gold surface more strongly that to neighboring layers, single layers of various van der Waals bonded chalcogenides, such as MoS,, WSe,, and Bi,Te3 with lateral sizes of several hundreds of microns were successfully exfoliated (Dobrik
FIGURE 1.9 Top: Schematic of micromechanical cleavage technique (the Scotch tape method) for producing few-layer stmctures. Top row: adhesive tape is used to cleave the top few layers from a bulk crystal. Bottom left the tape with removed flakes is then pressed against the substrate of choice. Bottom right: some flakes stay on the substrate, even on the removal of the tape (Source: Reprinted with permission from Novoselov (2011)). Bottom: Mechanically exfoliated single and few-layer MoS, nanosheets on 300 am SiO,/Si. Optical microscopy (a-d) (Source: Reprinted with permission fr om Li, Wu, Yin, and Zhang (2014). © American Chemical Society.
et al., 2015). Nanomechanical cleavage of MoS, was studied in-situ using transmission electron microscopy and layers with thicknesses varying from a ML to 23 layers were successfully cleaved (Tang et al., 1AD). Thermal annealing and evaporative thinning using a focused laser spot were also used for layer-by-layer thinning of MoS2 down to ML thickness by thermal ablation with micrometer-scale resolution (Castellanos-Gomez et al., 2012; Huang et al., 2014; Lu et al., 2013). Controlled MoS, layer etching using CF4 plasma has been reported (Jeon et al., 2015). The damage and fluorine contamination of the etched MoS, layer could be effectively removed by exposure to H,S.
Liquid Phase Exfoliation
Liquid exfoliation by direct ultrasonication, that was successfully used earlier to disperse graphene, was also employed to fabricate single-layer and multilayer nanosheets of a number of layered TMDCs, such as MoS,, WS2, MoSe2, NbSe2, TaSe2, MoTe2, MoTe,, and others (Coleman et al., 2011; Nicolosi et al, 2013) where the commercially purchased samples (in powder form) were initially sonicated in a number of solvents to form uniform dispersions; the dispersion mixture was then centrifuged to collect the exfoliated samples. Solvents having high dispersion coefficient, polarity, and hydrogen bonding capability are generally preferred as they facilitate the exfoliation process during sonication. Two mostly used solvents are N-methylpyrrolidone and isopropanol. These solvents allow the separation of van der Waals layers and prevent the agglomeration or re-stacking of produced layers by forming solvation shell. Samples such as boron nitride (BN), MoS2, and WS, have been prepared by this method using vacuum filtration or spraying techniques. Water has also been employed for this exfoliation technique on many occasions. In an aqueous medium, the samples are generally subject to vigorous sonication with the addition of a surfactant such as sodium cholate. The purpose of adding a surfactant is to coat the layered structures formed during sonication and protect them from getting re-stacked on each other (Smith et al., 2011). It may be worth noting that the dispersibility of exfoliated nanosheets varied only weakly between different TMDCs (Cunningham et al., 2012). The main challenge is to enhance the yield of the MLs and to maintain the lateral dimensions of the exfoliated sheets. Ion intercalation, such as lithium-intercalation or ultrasound-promoted hydration, is another approach, allowing fabrication of single-layer materials. The intercalation of TMDCs by ionic species allows the layers to be exfoliated in liquid (Dines, 1975; Joensen et al., 1986). In this exfoliation method, bulk TMCs, in powder form, is kept submerged in a lithium-based solvent such as n-butyllithium, for few days to facilitate the intercalation of lithium ions into the layers of TMCs. And then the dispersion mixture is exposed to water. The water reacts with the lithium between the layers; the process results in the formation of H, gas, which serves to separate the layers (Eda et al., 2012; Joensen et al., 1986). Such chemical exfoliation methods allow one to produce significant quantities of submicrometer-sized MLs (Tsai et al., 1997) but the resulting material differs structurally and electronically from the source bulk. For example, on exfoliation, the semi-metallic Mos, may change to metallic MoS2, as the coordination sphere of Mo shifts from trigonal prismatic (2H) to octahedral (IT) phase. However, annealing the IT phase at 300°C can reverse the phase to 2H. Lithium-based chemical exfoliation has been demonstrated for various TMDCs, in particular MoS„ WS„ MoSe2, and SnS2 (Gordon et al., 2002; Kinnayer et al., 2007). This method was also used to exfoliate topological insulators such as Bi2S3 and Bi,Te3 (Huang et al., 2009). An effective method for mass production of exfoliated TMD nanosheets is the ultrasound-promoted hydration of lithium-intercalated compounds. When the bulk TMCs are subject to exfoliation using organolithium compounds, intermediates are formed with the reduced phase, Li..MXn. and expanded lattice. If this intermediate phase is then treated in an aqueous medium (through ultrasound-assisted hydration process) (Dines, 1975; Frey et al., 2003; Tsai et al., 1997). Therefore, the important step in the formation of the reduced phase, i.e., LLMX,. tuning which can provide vital control over the quality of the exfoliated layers.
TMDC nanosheets can also be exfoliated by thermal cyclings, such as rapid freezing (30 s in a liquid nitrogen bath) and heating (20 min in an oil bath at 60°C), of hydrated TMDC powder in water (Chakravarty and Late, 2015). The lithiation process can also be carried out in an alternative maimer that uses an electrochemical cell with a lithium foil anode and TMDC- containing cathode (Figure 1.10) (Li et al., 2011; Zeng et al., 2012). As the intercalation occurs while a galvanic discharge is occurring in the electrochemical cell, the degree of lithiation can be monitored and controlled. The resulting Li-intercalated material is exfoliated by ultrasonication in water as before, yielding ML TMDC nanosheets.
FIGURE 1.10 The electrochemical lithium intercalation process to produce 2D nauosheets from the layered bulk material (MN = BN, metal selenides, or metal tellurides in LisMN). Source: Reprinted with permission Zeng et al. (2012). © John Wiley and Sons.
This technique was first reported in the case of MoS,, WS2, TiS,, TaS,, ZrS2 and graphene,(Li et al., 2011) and later extended for BN, NbSe,, WSe,, Sb,Se3, and Bi,Te3 (Zeng et al., 2012). This method is advantageous, considering the fact that it requires only a few hours to accomplish Li intercalation, as compared to longer time duration required in the case of ?/-butyl assisted method. During the whole experimental process, the lithium ions fulfill several important functions. First, the Li+ ions are inserted into the interlayer space of the layered bulk material, which expands the interlayer distance and weakens the van der Waals interactions between the layers. Second, the inserted Li+ ions are subsequently reduced to Li by accepting electrons during the discharge process. The metallic Li can react with water to form LiOH and produce H, gas (apparently, bubbles were observed during the experiments). The generated H, gas pushes the layers further apart. Under vigorous agitation by sonication, well-dispersed 2D nanosheets can be thus obtained (Zeng et al., 2012). For aqueous-based exfoliation procedures, lithium has been used extensively as compared to other alkali ions such as Na and K. Nevertheless, the atomic radii of Na and К are much larger than that of Li, and their reactivity towards water is more too. Thus, these ions can get intercalated into the layers of TMCs and expand them along the c-axis with more effectiveness. Taking this cue, naphthalenide adducts of Li, Na, and К was put to a comparison for their effectiveness in exfoliating MoS2 layers (Zheng et al., 2014).
Figure 1.11 shows the schematic diagram of the processing steps involved in obtaining well-dispersed samples of LTMDs. In the atypical procedure, bulk MoS, powders were first treated with hydrazine to loosen the layers taking the help of hydrothermal method (panel a) Decomposition and gasification of intercalated N,H4 molecules expands the MoS, sheets by more than 100 times compared to its original volume. In a second step, the expanded MoS, ciystal is intercalated by alkali naphthalenide solution (panel b).
FIGURE 1.11 (a) Bulk MoS, is pie-exfoliated by the decomposition products of N,H4. (b) Pre-exfoliated MoS, reacts with A+C10HS to form an intercalation sample, and then exfoliates to single-layer sheets in water, (c) Photograph of bulk single-crystal MoS,, d photograph of pre-exfoliated MoS,, (e) photograph of Na-exfoliated single-layer MoS, dispersion in water. Source: Reprinted with permission from Zheng et al. (2014).
In the final step, the alkali-ion intercalated MoS, was exfoliated by dispersing in water under constant ultrasonication. This method was tested successfiilly on a wide range of TMDCs (Zheng et al., 2014). A tandem molecular intercalation was proposed for producing single-layer TMDCs from multi-layer colloidal TMDC nanostructures in solution phase, where short ‘initiator’ molecules first intercalate into TMDCs to open up the interlayer gap, and the long ‘primaiy ’ molecules then bring the gap to full width so that a random mixture of intercalates overcomes the interlayer force (Jeong et al., 2015). With the appropriate intercalates, single-layer nanostructures of group IV (TiS„ ZrS,), group V (NbS,), and VI (WSe„ MoS,) TMDCs were successfully generated.
A typical arrangement for the electrochemical exfoliation of bulk MoS2 is shown in Figure 1.12a. ADC bias was applied between MoS: and the Pt wire for the electrochemical exfoliation, starting with a low positive bias to wet the bulk MoS, followed by a larger bias to exfoliate the crystal. As a result, many MoS, flakes dissociated from the bulk crystal and became suspended in the solution (panels b and c). The mechanism of electrochemical exfoliation of bulk MoS, crystals is described as follows (Figure 1.12e) (Liu et al., 2014). Fust, by applying a positive bias to the working electrode (WE), the
FIGURE 1.12 (a) Schematic illustration of the experimental setup for electrochemical
exfoliation of bulk MoS, crystal, (b) Photograph of a bulk MoS, crystal held by a Pt clamp before exfoliation, (c) Exfoliated MoS, flakes suspended in Na,S04 solution, (d) MoS, nanosheets dispersed in solution, (e) Schematic illustration for mechanism of electrochemical exfoliation of bulk MoS, crystal.
Source: Reprinted with permission from Liu et al. (2014).
oxidation of water produces -OH and -O radicals assembled around the bulk MoS, crystal. The radicals and/or SO,-4 anions insert themselves between the MoS, layers and weaken the van der Waals interactions between the layers. Second, oxidation of the radicals and/or anions leads to a release of O, and/or SO,, which causes the MoS, interlayers to greatly expand. Finally, MoS, flakes are detached from the bulk MoS, ciystal by the erupting gas and are then suspended in the solution. A major problem here is that bulk MoS, should be oxidized during electrochemical exfoliation, which may affect the exfoliated MoS, nanosheets, unless the conditions are, optimized (Liu et al., 2014).