Keywords

1 Introduction

Nanomaterials have been attracting the scientific world because of their unusual physiochemical characteristics due to their high aspect ratio, strange surface morphology, unique surface chemistry, and quantum-size effect, as compared to their bulk counterparts [1]. Based on their unique properties, they can be applied in a wide range of multifunctional application. In the last few decades, a large number of research works are published in the field of nanomaterials with a progressing span of about 30,000 publications per year. Dimensionality is one of the key factors to manipulate the nanomaterial properties. It is to be noted that the properties of nanomaterials are mostly attributed to their unique nanostructure and composition. Moreover, in terms of dimensions or confinement of electrons, nanomaterials have been categorized as zero (0D-), one (1D-), and two (2D-) nanostructures. For instance, the 0D nanomaterials (such as spherical nanoparticles and quantum dots) confine electrons in all three dimensions in the nanometre range. Similarly, with 1D and 2D structures, the electrons are confined in either one or two dimensions (2D) in the nanometre range [1]. In this chapter, we are interested to discuss different types of 2D nanomaterials along with their synthetic approaches, properties, and material characterizations.

In 2004, Novoselov and co-workers exfoliated a single-atom-thick layer of graphite using a mechanical cleavage method where the carbon atoms are arranged in a 2D hexagonal structure, which led to the discovery of Nobel Prize-winning graphene [2]. With the discovery of graphene, the scientific community has received a lot of attention to exploring the other potential 2D nanostructures. Therefore, after years of hard work, the scientists working in the field of 2D nanomaterials have found novel ways to synthesize 2D nanomaterials such as mechanical cleavage, ion-intercalation, liquid exfoliation, chemical vapour deposition (CVD), plasma-enhanced chemical vapour deposition (PECVD), hydrothermal syntheses [3,4,5,6]. For the synthesis of 2D nanomaterials, a large variety of materials are explored: from metals to semiconductors to insulators and even to superconductors which show remarkable properties with better performance than graphene. Based on the electron confinement in single-layered 2D nanomaterials, they show outstanding electronic and transport properties. In addition, the nanomaterials have a high surface area, owing to the exposure of edge sites, and flexibility in atomic level, high thermal endurance which made them suitable candidates for diverse applications such as in nano-electronics, optoelectronics, medical, civil fields and in space applications [5,6,7].

1.1 Type of 2D Nanomaterials

As already highlighted, graphene is known as a wonder nanomaterial consisting of crystalline carbon film and exhibits various unprecedented properties, such as ultrahigh carrier mobility (∼10,000 cm2 V–1 s–1), quantum hall effect, high specific surface area (2630 m2 g–1), optical transparency (∼97.7%), and excellent thermal conductivity (3000–5000 W-m–1 K–1) [2]. Inspired by the unexpected properties of graphene, the quest to explore other ultrathin 2D nanomaterials begins. Efforts are made by the researchers to exploit new 2D nanomaterials that possess similar features to that of graphene, yet capable of versatile properties. Among the family of new 2D materials originates the transition metal dichalcogenides (TMDs; e.g. MoS2, NbSe2, TaS2, WS2, MoSe2, WSe2, etc.), hexagonal boron nitride (h-BN), carbon nitride (CN), transition metal oxides, layered perovskites, metal–organic frameworks, black phosphorus (BP), and MXenes [7,8,9,10,11], as schematically presented in Fig. 1.

Fig. 1
figure 1

Schematic illustration of different kinds of typical ultrathin 2D nanomaterials, such as graphene, h-BN, TMDs, MXenes, TMOs (2D metal oxides), layered perovskite, and BP (phosphorene)

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Transition metal dichalcogenides (TMDs): TMD monolayers are semiconductor materials with atomically thin layers with a general representation of MX2; where M is a transition metal (Mo, W, Te, etc.), and X is a chalcogen atom (X = S, Se, Te, etc.). In a typical structure of MX2, one layer of M atoms is sandwich between two separate layers of X atoms to form X–M–X structure [10]. With this atomic arrangement, the crystal structure forms a honeycomb, hexagonal lattice. Usually, the M–X bonds (intralayer) between the transition metal and chalcogen atoms are known to be covalent bonds, whereas the individual MX2 layers are bonded together by van der Waals (vdW) (interlayer) forces which are considered as weak bonds. Moreover, the TMDs can form metal coordination such as trigonal prismatic or octahedral. This coordination of metals and also the order of staking between the monolayers describe the phase of TMD materials. Some most common phases are 1T, 2H, or 3R, where T, H, and R stand for tetragonal, hexagonal, and rhombohedral, while 1, 2, and 3 denote the number of X–M–X sandwiches per unit cell.

Hexagonal boron nitride (h-BN): h-BN is a layered material that has a similar structure to graphite, which in turn has similar properties like graphite. The crystal arrangement of h-BN consists of alternate boron and nitrogen atoms which is bonded with sp2 hybridization forming an atomic-level thick layer [11, 12]. Usually, the bond length among two consecutive boron and nitrogen atoms is 0.144 nm. h-BN exhibits covalent bonding within the plane. Irrespective of this, its interplane bonding is relatively weak due to vdW forces. In h-BN, the spacing between two successive layers is 0.334 nm which is analogous to graphene (0.333 nm). Thus, h-BN displays advantages for efficient electronic and optical applications.

Carbon nitride (CN): Graphitic carbon nitride (general formula g-C3N4) is one of the hardest and most stable compounds belonging to the library of 2D nanomaterials. The fundamental structure of C3N4 forms a polymeric stacked structure which is more like graphite containing sp2 hybridized carbon and nitrogen atoms [13]. It consists of tris-triazine-based patterns where the carbon to nitrogen ratio is typically ¾ with a small amount of hydrogen. Because of the presence of nitrogen (lone pair) and π-conjugation system with carbon pz-orbital, the C3N4 shows promising electronic properties. There are mainly two known structures of C3N4, one is based on heptazine, and the other is poly-triazine imide units based on which the reaction conditions, reactivities, and the associated properties can vary.

Transition metal oxides: The transition metal oxides from their bulk state can be transformed into atomically thin nanosheets, thus placing their stand as 2D nanomaterials [14]. It is already known that the bulk metal oxides can show unique combination of both physical and chemical properties with earth abundance. However, the 2D nanosheets of metal oxides provide even more unprecedented features such as larger surface area, improved active sites, and interplanar charge transport. There are various metal oxides found in nature, and to name a few are MoO3, TiO2, ZnO, Co3O4, V2O5, etc. Every 2D metal oxides have different crystal structures which will be discussed in detail in the succeeding sections of the chapter.

Layered perovskites: The dimensionality of the 3D perovskite structure when reduced to layered 2D form enables very interesting properties for diverse applications. In simple terms, the 2D perovskite resembles the slice cut from the 3D perovskite lattice. Different from the 3D perovskite with ABX3 structure (where a larger cation (A) simply fills the voids between BX6 octahedra), and in 2D perovskite, some larger cations are introduced into the structure with acts as spacers [15]. These spacers help to isolate the inorganic octahedra to form quantum well superlattices. Furthermore, these extra spacing cations induce asymmetric lattice structure, thereby providing an additional degree of freedom. Thus, the structural slicing along a definite crystallographic plane forms a layered material that is coupled by weak vdW interactions. The empirical formula of 2D perovskites is L2MX4, where L is the large monovalent organic cation/spacer, M the metal cation, and X the anion. Few examples of 2D-layered perovskites are Cs2PbI2Br2, (BA)2(MA)2Pb3I10, etc.

Metal–Organic frameworks: Metal–organic frameworks also form 2D structures which are considered as an attractive alternate crystalline porous material [16, 17]. It basically consists of metal bridging nodes and multi-podal organic ligands that are bonded together. This type of bonding usually forms through basic coordination chemistry. In general, the metal node is coupled to the organic ligand by a coordination bond; however, the layers of the metal–organic framework are linked by weak vdW forces.

Elemental 2D nanomaterials: Elemental 2D nanomaterials are attracting great attention with the experimental demonstrations of borophene nanosheets belonging to the group III. Boron forms almost 16 number of allotropes through complex B–B bonds, out of which only three forms are thermodynamically stable (α-rhombohedral, β-rhombohedral, γ-orthorhombic, and γ-tetragonal) [18]. These allotropes consist of the icosahedral B12 units as the building blocks. Notable structure among the borophene is the B36, which has a bowl-shaped cluster with a periodic hexagonal holes arrangement forming a triangular lattice. The hexagonal arrangements of atoms can be visualized as graphene-like nanosheets [18, 19].

Another interesting 2D nanomaterials are silicene, germanene, and stanene belonging to the group IV elements [19]. Silicene is derived from silicon which comprises a buckled sheet and can host non-trivial electronic states, spin-polarized edges, and a tunable bandgap that allows for application in quantum information. Similarly, germanene is also a buckled monolayer of germanium that forms a nearly flat honeycomb nanosheets [18]. Germanene shows strong spin–orbit coupling for topological insulator properties.  Stanene, on the other hand, is derived from tin (α-Sn).  Stanene is epitaxially grown on various substrates; however, its structure is not well-defined [18, 19].

From the group V elements, phosphorene or black phosphorus (BP) is an allotrope of phosphorous which is known to be thermodynamically stable at room temperature (RT). It also emerges as one of the novel 2D semiconducting materials which has similar properties and colour of graphite. Typically, BP has an orthorhombic crystal structure, closely analogous to graphene. In the crystal arrangement, the phosphorous atoms are organized in a honeycomb lattice with puckered double layers. BP monolayer has sp3 hybridization where one phosphorous atom is covalently bonded to three other adjacent phosphorous atoms having a bond length of 2.18 Å [20]. In addition, it has one lone pair of electrons, thus forming a quadrangular pyramid structure. The adjacent layers of phosphorous atoms interact by weak vdW interactions where the distance between two layers is about 0.5 nm. Similarly, arsenene also belongs to group V that has a single buckled honeycomb 2D layer of arsenic. Arsenene has an indirect bandgap of 2.49 eV and a high charge carrier mobility. Another 2D structures are antimonene and bismuthene which are extracted buckled honeycomb network of antimony and bismuth [19]. Antimonene has high carrier mobility and excellent thermal conductivity. Bismuthene is extensively used in topological insulators through the reduction of bismuth [ref]. Notably, various structures and properties of BP-like puckered structures (α-phase) of arsenene, antimonene, and bismuthene have been predicted [18,19,20]. However, fabrication of monolayer α-phase is challenging mainly due of the lack of layered allotropes.

MXenes: With the expansion of the family of 2D nanomaterials, the derivatives of transition metals using carbides, carbonitrides, and nitrides lead to the discovery of MAX phases where M stands for transition metal (Ti, Zr, Cr, etc.), A is an element from A-group mainly IIIA, IVA (Ga, Pb, Al, etc.), and X is either carbon or nitrogen. By introducing selective etching of the A element from the MAX phases, MXenes are created. MXenes are considered as layered solids linked by strong metallic, ionic, and covalent bonds. Having a layered hexagonal structure, they belong to space group P63/mmc symmetry. MXenes have a general formula Mn+1AXn, with n = 1–3, and thereby, the MXene sheets consist of 3, 5, or 7 atomic layers constructing M2X, M3X2, and M4X3, respectively [9]. Some examples of MXenes are Ti2C, Ti3C2, Mo2Ga2C, Nb4C3, etc.

2 Synthesis of 2D Nanomaterials

The 2D nanomaterials possess both similarities and differences in their fundamental crystalline features compared to their 3D counterparts. Therefore, to maintain their unique structural/crystalline features and simultaneously retaining their outstanding properties, atomically thin 2D nanomaterials have become the forefront in condensed matter physics, materials science, and nanotechnology. In literature, there are numerous methods available for the synthesis of 2D nanomaterials. However, before addressing the individual experimental techniques, it is worth mentioning that the nanoscale synthesis methods are broadly categoried into two types: bottom-up approach and top-down approach (Fig. 2) [21].

Fig. 2
figure 2

copyright permission from [5,6,7,8, 10]

Different synthesis techniques for the growth of 2D nanomaterials: top-down and bottom-up approaches. a Mechanical cleavage method (1) press the adhesive tape against the 2D crystals, (2) few layers are attached to the tape, (3) the tape with crystals is pressed against a surface, (4) upon peeling, the bottom layer left on the substrate, b hydrothermal method, c CVD technique, d sonication derived exfoliation via (1) the addition of exfoliating/stabilizing agents, (2) sonication, and (3)  exfoliated 2D nanosheets. e Ion-intercalation method. Reprinted with

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2.1 Bottom-Up Approach

The bottom-up approach is also referred as the gathering-up method in which the 2D nanomaterials can be synthesized from atomic or molecular precursors in the form of nanoparticles or nanocrystals that are allowed to grow in size after undergoing certain chemical reactions. The nanoscale particles thus self-assembled into larger complex substances or can be supplemented to any substrate of interest [21, 22]. The bottom-up approach is mainly desired for achieving controlled size, shape, composition, sequential stacking arrangement, and stable chemical structure. There are various synthetic methods used to prepare 2D nanomaterials using the bottom-up approach such as hydrothermal or solvo-thermal method, wet chemical synthesis, CVD, PECVD, organic ligand-assisted synthesis, interface-assisted synthesis, and template-assisted synthesis [23, 24]. Here, we have discussed the most regularly used techniques.

Hydrothermal method: This is one of the most common and widely used synthesis methods of 2D nanomaterials. It is cleared from the name itself that this technique deals with water (hydro) and heat (thermal). This means that this method is somewhat analogous to wet thermal synthesis. Although there are few additional requirements in the hydrothermal method which are the high temperature and pressure. Typically, the hydrothermal reaction is carried out inside an autoclave (vessel) with a Teflon liner. At first, the precursor materials are mixed thoroughly in an appropriate concentration and then transferred to the autoclave vessel. The autoclave vessel is designed in such a manner so that it can easily work under high-pressure conditions. Finally, the precursor-filled vessel is placed in a heating furnace, setting a specific temperature and time for the reaction to start till completion. Here, the solvents serve as the catalyst to initiate the reaction and help the growth of nanostructures [25]. This technique has several benefits such as high product yield, controlled size/shape, high-quality crystals, and most importantly, the low-cost instrumentation with environment-friendly synthesis. Yin et al. have presented a novel hydrothermal strategy to synthesize MoSe2 nanosheets [26]. This synthetic approach provides the synergistic regulation of both crystal phase (1T) and disorder engineering. In addition, one modified approach using the hydrothermal technique is reported by Dai et al., where graphene-like MoSe2 nanosheets are synthesized under the synergy of graphene and polyvinylpyrrolidone [27]. Moreover, Alam et al. have synthesized metal oxide-based 2D nanomaterials using Bi-TiO2 nanotube/graphene nanocomposites through simple hydrothermal synthesis [28]. Similarly, various 2D nanosheets of metal oxides such as Ga2O3, ZnGa2O4, and MnGa2O4 are prepared by hydrothermal synthesis [29]. These nanosheets exhibit a triangular/hexagonal configuration with ultrathin thickness. In addition, Peng et al. demonstrated the use of a modified hydrothermal route using some basic etching agent to synthesize 2D MXene (Ti3C2) [30]. They have also extended their route to other MXene such as Nb2C. Subsequently, to boost the material yield of MXenes, Han et al. have adopted a facile hydrothermal-assisted intercalation approach to form 2D Ti3C2Tx and achieved 74% yield which was conventionally limited to 20% [31]. Hydrothermal synthesis is also used to prepare 2D nanosheets of boron nitride. Xie et al. reported the hydrothermal exfoliation method where the bulk h-BN undergoes expansion with the insertion of Li+ and then exfoliated into ultrathin 2D nanosheets [32]. Catalyst-free g-C3N4 is also prepared by the low-cost hydrothermal approach [33].

Chemical vapour deposition (CVD) method: As the name suggests, CVD is a gas phase deposition method for the preparation of thin films over any desired substrate. Here, the precursors (volatile or viscous liquids) are injected into the vacuumed sealed reaction chamber in the form of vapours/gases and are allowed to go through some specific chemical reactions at a specific ambient temperature. Consequently, the placed substrates get coated when the reaction products of the precursors are assembled onto it [23, 34]. There are various synthesis parameters to take care of while performing the deposition such as process temperature, pressure, substrate temperature, and gas mixture composition. Therefore, simply controlling these parameters, it is possible to synthesize good quality thin films with high purity. This technique has some special advantages: for instance, low film porosity, high purity, and outstanding stability at air ambient comprehensively adopted in industries. Based on the CVD method, numerous 2D nanomaterials are synthesized for targeted applications. McCreary et al. reported the synthesis of large-area monolayers of WS2 using CVD [35]. Likewise, Dankert et al. have worked on the fabrication of all CVD-based heterostructures with h-BN/graphene/h-BN configuration for developing high-performance Hall sensors [36]. As one kind of CVD technique, the researchers have also explored the PECVD technique to grow CN films using methane (CH4) and N2 gases [37]. Again, adopting an in-situ CVD approach, large-area 2D BP is grown with an average area of > 3 µm2 and about four layers of thickness [38]. A similar effort has been done by other groups to grow TMDs (WS2) atomic layer on h-BN film by CVD technique [39].

Interface-assisted synthesis: Basically, an interface is referred as any flat or curved space (more precisely can be known as any phase boundary) between two dissimilar materials. Likewise, the interface formed between air and matter or between vacuum and matter is called the surface. Typically, the thickness of any interface ranges from angstrom (Å) to nanometres to few micrometres. Therefore, once the ratio of area to thickness in any interface is high enough, it is called a 2D interface. In the field of materials science, this 2D interface is highly active for multiple reactions as compared to the bulk phase. Some common interfaces that are involved in chemical reactions include the air/liquid, liquid/liquid, solid/liquid, and vacuum/solid interfaces. These types of 2D interfaces provide space to the precursors to gather and induce the growth process or nucleation in a confined 2D space. This confined reaction in 2D space plays an important role in controlling the material properties of the final product [40]. Therefore, as one kind of bottom-up approach, there are various interface-assisted synthetic approaches reported in the literature for the development of 2D nanomaterials. Considering the synthesis of graphene (a star material in the field of materials science), the gas/solid or the vacuum/solid interface is a popular strategy that can be done using CVD. In CVD, a selective solid substrate is kept in a vacuumed/air chamber, and the gases of interest (say CH4 for graphene) are inserted into the chamber which is allowed to react on the substrate. Pollard et al. confirmed the growth of graphene on a Ni film (Ni is used as a catalyst) deposited on the SiO2/Si substrate using CVD [41]. Similarly, using CVD with a vapour/solid interface, h-BN is also synthesized by Shi et al. [42]. Interfacial synthesis strategies towards the preparation of TMDs have also been explored. Transition metal selenides such as TiSe2, NbSe2, and TaSe2 are synthesized by surfactant lamellar templating which acts as efficient 2D catalysts. These 2D structures feature the thickness from single sheets to tens of nanometres [43]. Other noble 2D metal oxides such as SnO2, CuO, and In2O3 are developed using surfactants. For the growth of 2D metal oxides using bottom-up approach from molecular precursors, Sun et al. have addressed the surfactant self-assembly, where the surfactants serve as the agents for structure design and help to confine the growth along the desired 2D direction [44]. In addition, an interesting 2D metal–oxide framework nanosheets are demonstrated by Nishihara and co-workers. At the air/water interface, they have tried to synthesize a single-layer nickel bis(dithiolene) nanosheets under atmospheric pressure [45]. Moreover, with regards to the other 2D nanomaterials such as MXenes, CN, and BP, the current synthesis methods are primly focused on the physical and chemical exfoliation of their bulk structures. Thus, the growth of single- to few-layered structures through the interface-assisted synthesis is still under progressive development.

2.2 Top-Down Method

In contrast to the bottom-up approach, the top-down approach is more complex and requires sophistication. The top-down approach is also referred as the destructive method where the nanostructures are formed by removing the building blocks from the matter or cutting/sizing the solid crystal planes [21]. Particularly considering the synthesis of 2D nanomaterials, the top-down approach involves the breaking of interlayer spacing of a 3D matter to prepare atomically thin layers. Again, the top-down method is grouped into various types such as exfoliation method, chemical etching, sputtering, nano-lithography, and laser ablation. Here, we have discussed some of the widely used techniques:

Exfoliation method: The exfoliation method is categorized into two types: chemical exfoliation and physical exfoliation. Chemical exfoliation is resulted from the thinning of a layered crystal down to atomic-level layers through appropriate chemical routes involving intercalants [46] to form 2D-layered structures. A large number of intercalants are used for chemical exfoliation such as acids/bases (HSO4−, SO22−, and KOH), inorganic salts (Li+, Na+), oxidizing agents (hydrogen peroxide and hydroxyl radicals), and functional molecules (NH+, pyrene sulfonic acid). Diazonium salts are effectively used to chemically exfoliate the semiconducting bulk MoS2 in the 2H phase [47]. Similarly, some other bulk chalcogenides such as Bi2S3 and Sb2S3 are also exfoliated into 2D nanosheets using this chemistry [48, 49]. On the other hand, one report addresses that with aryldiazonium modification, it is possible to exfoliate bulk BP without undergoing any pre-treatment and passivation of the surface to form 2D nanosheets which can be applied in field-effect transistors (FET) [50]. Composites of graphene oxide/manganese phosphate are also realized by the chemical exfoliation process through Hummer’s method as reported by Yuan et al. [51]. The liquid exfoliation method is also explored by many researchers for the synthesis of 2D metal oxides. This process is associated with gentle cutting of the bulk interlayers where different organic cations such as TBA and ammonium ions are used as the intercalant agents [52]. Chemical exfoliation of MAX phases into 2D MXenes is also highlighted by the research community. Khazaei et al. have utilized a series of first-principle calculations based on DFT and studied the exfoliation energies, forces, bond strengths, and electronic structures of the MAX phase. Based on their DFT calculations, around 37 MAX phases are proposed for successful exfoliation into 2D MXenes such as Ti3C2, Ti4C3, Zr2C, Hf2C, V3C2, V4C3, Mo2C, and so on [53]. From the above-mentioned examples, now it is clear that with chemical exfoliation the chemical structure of 2D nanomaterials can be tuned through some appropriate reactions. During the chemical exfoliation process, sometimes the 2D nanomaterials suffer from loss of their inherent properties causing rapid degradation.

Physical exfoliation, on the other hand, can preserve the important properties of exfoliated 2D nanomaterials without experiencing much degradation. Physical exfoliation also offers large-scale production suitable for all practical applications. This type of exfoliation is mainly achieved through some external driving forces such as sonication, wet/dry ball milling, shear mixing, using some supercritical fluids, polar/non-polar solvents, and stabilizers. Considering the simplest case of ultrasonication, it has been reported by Kim et al. that any layered bulk material such as graphene, MoS2, and h-BN can be exfoliated by controlling the temperature in an ultrasonic bath [54]. The as-exfoliated 2D materials, for instance, h-BN display an alternating charge distribution with a strong polarity across the boron and nitrogen termination edges. However, MoS2 and MoSe2 provide moderate polarity, where the surface has a negative charge with the sulphur atoms and counter-charges inside the molybdenum atoms. With ultrasonication, the 2D nanomaterials often suffer from the introduction of undesired defects. Therefore, the use of supercritical fluid is another approach to physical exfoliation [55]. As one of the most common supercritical fluids, CO2 presents a solvent-like behaviour which can dissolve non-polar chemical species. Using CO2, thick nanosheets of more than ten layers can be formed [56]. A molecular dynamics study is conducted for stabilizing MXene (Ti2CO2) structures using supercritical CO2 as reported by Khaledialidusti et al. [57]. In addition, ball milling is also one type of physical exfoliation technique which is used to prepare MXenes (Ti2C2Tx). After ball grinding, the Ti3AlC2 size is found to be uniform and can be tuned from 4.488 to 1.454 μm, with a well-defined 2D nanosheets structure [58]. Similarly, g-C3N4 powders are also synthesized by ball milling from amorphous carbon at high temperatures [59]. Graphene, TMDs, and h-BN synthesis with solvent-stabilizer exfoliation is also addressed using PVA-assisted shear-exfoliation and using chloroform/acetonitrile and IPA/water [60].

Sputtering: Sputtering is a comprehensively used industrial-based technique that provides large-scale high purity production of diverse materials maintaining high quality and controllability. With this technique, it is possible to use any kind of substrate and even applicable to insulating material deposition. In general, sputtering is performed under a vacuum with a target material to be sputtered. Generally, an inert gas (or a reactive gas) is used as a carrier gas to initiate the plasma glow upon applied bias. The ionized gases are then accelerated to the target material in the presence of an electric field and thus bombards the target surface with high kinetic energy, thereby ejection of the target atoms. Those target atoms are then self-assembled or accumulated on the substrates to form the smooth and uniform coating [61]. Various 2D nanomaterials are synthesized by sputtering. Rigi et al. have demonstrated a RF magnetron sputtering method to produce MoS2 layers [62]. For the experiment molybdenum target was used under sulphur environment using an in-situ effusion cell. A highly pure 2H-MoS2 phase is obtained using RF magnetron sputtering. Likewise, thin MoSe2 interlayer is prepared by sputtering and selenization process by pressure variation and applied for efficient Cu(In, Ga)Se2 solar cell (PCE = 10.8%) [63]. Moreover, high-quality boron nitride (BN) films are also deposited by magnetron sputtering as addressed by Sutter et al. [64]. Here, the boron is sputtered under N2 and Ar environment, and the thickness of the deposited film is carefully controlled. Ensuring effective substrate temperature, two atomic layers of BN are successfully deposited by magnetron sputtering. Li et al. have reported an ionized magnetron sputtering method to deposit amorphous CN [65]. High purity graphite is used as the sputtering target with an inductively coupled plasma assembly between the target and the substrate under Ar/N2 mixture gases. Within the N2 to C ratio of 0.3–0.4, hardness up to 16–17 GPa is achieved. Moreover, Wang et al. using magnetron sputtering tried to deposit MXene with molybdenum carbide (Mo2C) structure to be applicable in solid-state Q-switched pulsed laser generation [66]. The overall summary of the different synthetic approaches for the preparation of 2D nanomaterials including their advantages and challenges is tabulated in Table 1.

Table 1 Summary of various synthesis techniques to grow 2D nanomaterials with their achievements and challenges
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3 Properties and Characterizations of 2D Nanomaterials

The 2D nanomaterials based on their unique structures and wide range of synthetic approaches emerge as one of the novel classes of nanoscale materials. With the unique structural constructions of various 2D nanomaterials, they show interesting properties like unprecedented optical or photonic properties and tunable morphological features. In addition, the 2D nanomaterials also offer unique electrical transport properties based on their dimensional restrictions. Moreover, the thermal, mechanical, and magnetic properties are also explored for some targeted applications with high stability (Fig. 3). Therefore, concerning the new synthesis approaches and vibrant properties, the 2D nanomaterials are realized in various applications related to material sciences such as in energy conversion (solar cells, water electrolysis, photocatalysis, thermoelectric devices, etc.) and energy storage devices (supercapacitors, batteries, etc.) [4,5,6,7,8,9,10,11]. In this section, we will elaborate on the individual properties shown by various 2D nanomaterials.

Fig. 3
figure 3

Different properties of 2D nanomaterials with their corresponding unique characteristic features

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3.1 Structural Properties

Geim and Novoselov won the Nobel Prize in 2004 for the invention of graphene which is an important class of 2D nanomaterial [2]. Graphene holds a hexagonal structure with sp2 hybridized carbon atoms. The structure has alternate single and double bonds (i.e. a conjugated structure) with p-orbital overlapping and electron delocalization. Because of this structural arrangement, graphene holds a very stable crystalline form and offers unique properties such as high surface area, outstanding catalytic features, high optical transparency, and so on. Graphene has shown its potential as a promising candidate for diverse applications, for instance, in optoelectronics, supercapacitors, drug delivery, biosensors, and image sensors. Regardless of consuming numerous advantages and applications, graphene has some shortcomings too. For example, the material is insoluble and infusible in nature. Therefore, for full utilization of graphene, some structural modifications are always required such as modifications in the basal plane, edges, and surface functionalization.

Now, expanding the portfolio of 2D nanomaterials from graphene, currently other new 2D materials such as TMDs, h-BN, g-C3N4, MXenes, and black phosphorous are in the forefront [4,5,6,7,8,9,10]. The sandwiched structured TMDs with X–M–X structure exhibit semiconducting properties with a tuneable bandgap which makes them a suitable candidate for optoelectronic devices [9]. Moreover, based on its unique layered structures, it is also used as an efficient catalyst material for applications in electrochemical cells [11]. SnS2 belongs to the class of TMDs which crystallize in hexagonal CdI2 lattice structure. It shows n-type semiconducting characteristics and is known to have a wide bandgap (2.03–2.4 eV) [67]. Its strong anisotropy makes it possible for its use in efficient holographic recording systems. Moreover, other TMDs such as MoS2, MoSe2, WS2, and TaS2 have out-of-plane mirror symmetry and in-plane inversion symmetry, owing to which they show complementary characteristics to that of graphene [9]. Also, in some cases, even these TMDs surpass the performance of graphene. The main uniqueness of these TMDs is their strong spin–orbit interaction. The most studied MoS2 is a semiconductor with a 2H phase and tuneable bandgap from 1.2 to 1.9 eV. In addition, the MoS2 monolayers have in-plane Young’s modulus of 200–300 GPa. Another special parameter is its high mobility (> 190 cm2 V−1 s−1), high on/off current ratio (108), and polarization properties. As such, MoS2 shows potential applications in several fields such as nano-electronics, solar cells, chemo-sensing, energy storage, and catalysis [9, 68]. Tran et al. recently announced a new member of the MXene family: V4C3Tx which is prepared by means of chemical exfoliation of the MAX phase (V4AlC3) using aqueous hydrofluoric acid [69]. The successfully exfoliated MXene V4C3Tx is confirmed from the SEM morphology and STEM characterizations. In parallel, Rosen’s group has addressed the synthesis of quaternary MAX solution with (Nb2/3Sc1/3)2AlC structure. They have selectively etched both Al and Sc atoms to produce Nb1.33CTx MXene, which is confirmed from XRD, XPS, and STEM [70]. Likewise, h-BN also belongs to the family of 2D nanomaterials which has a noteworthy structural resemblance to graphene. The 2D h-BN comprises both armchair and zigzag edge-terminated structures, where the layers are coupled to one another by the relatively weak vdW forces. The h-BN offers a high surface area and improved stability [12]. Silva et al. have demonstrated the synthesis of nanostructured BN through thermal CVD at 1150 °C. For this, iron compound (FeS/Fe2O3) is used as the catalyst on Al2O3 nanostructures [71].

3.2 Optical and Photonic Properties

As already highlighted, the 2D nanomaterials provide diverse properties suitable for a broad spectrum of applications. One important property is the optical/photonic response of the 2D nanomaterials. The optical characteristics of materials are mostly determined by the electronic band structure. The precise engineering of the electronic bands in 2D nanomaterials yields useful optical properties. It is well known that every 2D nanostructured material shows different optical bandgaps based on their individual structure–property relationship. The layered 2D halide perovskites show unprecedented optical properties with a tunable bandgap suitable for optoelectronic applications such as photodetectors, solar cells, and light-emitting diodes. [72]. MA3Bi2I9, Cs2PbI2Cl2, and (BA)2(MA)2Pb3I10 have a 2D-layered structure with bandgaps ranging from 1.5 to 1.9 eV. As one of the typical layered perovskite derivatives, these are used in solar cells and photodetectors [73]. Moreover, some 2D nanomaterials have large electronic bandgaps such as TMDs, h-BN, and MXenes. In particular, significant changes occur in the optical properties of TMDs due to the existence of indirect-to-direct bandgap transition. This depends on varying the thicknesses of MoS2 layers. Monolayered MoS2 with an energy bandgap of 1.8 eV can detect green light, whereas a triple-layered MoS2 with a bandgap of 1.35 eV responds towards red light [74]. Moreover, h-BN and BP are also being investigated extensively for their photonic and optoelectronic properties. Brar et al. have demonstrated the surface phonon–plasmon–polariton modes in a heterostructure based on graphene and h-BN. They have shown experimentally that the plasmon mode of graphene is split into two modes which display anti-crossing behaviour near the energy of h-BN optical phonon at 1370 cm−1 [75]. Preparation of g-C3N4 nanosheets incorporating plasmonic silver as an efficient photocatalyst is also explored for enhanced visible-light photocatalysis experiments. Deng et al. have studied the improved photocatalytic activity of plasmonic Ag and N2 doped graphene QDs which are co-decorated on g-C3N4 nanosheets [76]. The experimental findings reveal an improved photocatalytic activity with 92.8% removal efficacy under white light and NIR irradiation. Plasmonic photodetection of MXenes is also reported based on its atomically thin layers. Velusamy et al. have reported Mo2CTx MXene thin films which are successfully deposited on paper substrates. The as-fabricated photodetectors with this MXene structure exhibit extended photoresponse within 400–800 nm along with a high responsivity of 9 AW−1 and detectivity of 5 × 1011 Jones. It also exhibits reproducible photo-switching featured at a wavelength of 660 nm [77]. Moreover, a photodetector based on black phosphorous is also reported by Engel et al. for high-speed imaging [78]. Here, a multi-layered BP is capable of acquiring high contrast in the visible and infrared region of the electromagnetic spectrum showing its applicability for broadband optical detection.

3.3 Electrical Properties

Electronic/electrical property is influenced by the presence of localized or delocalized transport of electrons in a solid material. Based on the electronic properties of various materials, the economy of numerous industries has improved by adopting state-of-the-art fabrication technologies. For the case of 2D nanomaterials, the electrons or holes are restricted to occupy the quantized energy levels in one spatial dimension. Based on this, various important electronic properties are arising related to the energy level, transport, and phonon scattering and excitation in 2D nanomaterials.

Numerous heterostructures have been designed based on graphene for bandgap engineering such as graphene/h-BN, graphene/ZnO, and graphene/MoS2 [79, 80]. In general, TMDs hold a vast range of electronic properties, from semi-metals to semiconductors to insulators. Xiong et al. reported the lithium intercalation in MoS2 [81]. Because of Li insertion into the interlayer spacing, the electrical conductivity enhanced 200 times. Likewise, electrical transport in various TMDs has been investigated such as in 2H-NbS2, -NbSe2, -TaS2, and TaSe2, respectively. Here, to investigate the carrier scattering mechanisms, the charge density wave (CDW) system is used. Based on this, the measurements of the resistivity and the Hall coefficient of 2H-TMDs are carried out between 4.2 and 300 K [82]. The resistivity of 2H–NbS2 displays no CDW transition. On contrary, 2H–TaSe2 shows the highest CDW transition and along with several anomalous features. The electrical properties of MXene structure Ti3C2Tx monolayers are investigated by Miranda et al. [83]. They have demonstrated the metallic nature of MXene with a high free carrier density of 8 ± 3 × 1021 cm−3 and high mobility of 0.7 ± 0.2 cm2 V−1 s−1. Based on the electrical performance of MXenes, they have been successfully utilized as promising candidates for energy storage applications, for example, in Na, Li, K ion batteries, supercapacitors, and fuel cells. As a 2D equivalent of graphene, the electrical properties of h-BN are also studied both individually and by forming heterostructure of graphene and h-BN [84]. In general, h-BN is considered as an insulator with a wide bandgap; however, forming the heterostructure with graphene (graphene/h-BN) for transistor application, the electron mobility and the drain current switching ratios are as high as 573 cm2 V−1 s−1 and −2 × 1011 cm−2 [85]. Moreover, the electrical properties of sputter-deposited CN thin films are also studied by Broitman et al. [86]. They have reported that by increasing the N2 content during sputtering, the resistivity decreases from 4 × 10–2 Ω-cm to 4 × 10–3 Ω-cm. The electrical conductivity of BP has been measured by Keyes et al. [87]. They observed p-type conductivity under low temperatures. The electron and hole mobilities at room temperature are reported to be 350 and 220 cm2 V−1 s−1, respectively.

3.4 Thermal Properties

Thermal property management is an essential subject for designing robust electronic devices. 2D nanomaterials based on the thermoelectric effects can directly convert heat into electricity for harnessing waste heat. For this, mainly the Seebeck effect is used to modulate the conversion of heat into voltage [88]. Numerous studies were performed to achieve appreciable Seebeck coefficients. For instance, Hippalgaonkar et al. and Hewitt et al. have reported the Seebeck coefficients of MoS2 and Sb2Te3 to be 8.5 mW-m−1 K−1 and 371 µW-m−1 K−1, respectively [89, 90]. In addition, among all the TMDs, WS2 based on the Boltzmann transport equation provides the highest thermal conductivity of 142 Wm−1 K−1 followed by MoS2 and MoSe2 with 103 and 54 Wm−1 K−1 [91]. It has been addressed that the heterostructures of 2D nanomaterials provide an ideal platform to study interfacial heat transport. The interface thermal conductance of MoS2 on Au substrate is as high as 221 MW-m−2 K−1 [92]. Moreover, polymeric carbon nitride (PCN) is investigated for thermoelectric performance using molecular dynamic simulations. It is found that PCN has a high figure-of-merit, ZT (ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electronic conductivity, T is the absolute temperature, and κ is the thermal conductivity) of 0.52 at 300 K which contributes to n-type thermoelectric group materials [93]. Heterojunction devices based on graphene/h-BN also offer a high thermoelectric power factor of 10.35 W-m−2 K−1 [94]. Introduction of MXene (Ti3C2Tx) into (Bi, Sb)2Te3 matrix also provides improved thermoelectric performance with ZT of 1.3 within 300–475 K towards high thermoelectric conversion efficiency [95].

3.5 Mechanical and Magnetic Properties

Mechanical properties of 2D nanomaterials play an important role in various applications. Some mechanical properties include the fracture strengths, Young’s modulus, elasticity, etc. [96]. One of the ways to control the mechanical properties of 2D nanomaterials is based on defect engineering, which helps to enhance the toughness of classical materials from metals to semiconductors to insulators [97].

Magnetic properties of 2D nanomaterials are of utmost importance which can find interest in applications related to electric motors, computers, and medical diagnosis. For example, ferromagnetism is considered as one of the intrinsic properties of atomically thin layered 2D materials at room temperature [98]. This property has also shown a broader prospect for nano-device design. In particular, for spintronic applications, the intrinsic magnetic orders in ultrathin 2D nanomaterials have been extensively studied [99]. Sanikop et al. have tried to tailor the magnetically active sites of MoS2 nanosheets for spintronic applications [100]. For this, defect-density controlled 2H phase of MoS2 nanosheets is prepared at 500–900 °C which shows a ferromagnetic-like transition at 120 K. Likewise, Du et al. have elaborated the first-principle prediction of metal-free magnetism in g-C3N4 [101]. The ferromagnetic ground state displayed by the g-C4N3 also possesses an intrinsic half-metallicity. Kumar et al. have demonstrated the intrinsic ferromagnetism in Mn2NTx, Ti2NOx, and Cr2NOx, MXene structures [102]. High magnetic moments, high Curie temperature (1877 K), and robust ferromagnetism are found in these MXenes for spintronic applications. Furthermore, in the heterostructure of Ni(OH)2 and h-BN, a larger magnetic moment with ferromagnetic coupling is found [40]. Furthermore, ferromagnetic 2D nanomaterials with superior electronic and optical properties are used for the construction of compact magnetic, magneto-electronic, and magneto-optical devices. Table 2 provides an overview of all the unique properties of various 2D nanomaterials.

Table 2 Summary of different properties of various 2D nanomaterials
Full size table

4 Future Aspects and Conclusions

This chapter offers an overview of the fundamentals of 2D nanomaterials including their structure, types, synthesis, and properties. It is now well established that 2D nanomaterials are the special class of materials which open up a new avenue of diverse applications including energy conversion, storage, and in the environment and biomedical fields. We have mainly highlighted the 2D nanomaterials based on TMDs (MoS2, WSe2, MoSe2, etc.), BP, g-C3N4, h-BN, MXenes, 2D metal oxides, and layered perovskites. The 2D nanomaterials can be synthesized from various routes, among which we have discussed in detail the bottom-up and top-down approaches. Finally, numerous interesting properties including structural, optical, electrical, mechanical, thermal, and magnetic properties are elaborated with examples of 2D nanomaterials. Based on the special properties of 2D nanomaterials, significant applications are highlighted related to energy conversion technologies suitable for energy harvesting and energy storage devices. However, the commercialization of 2D nanomaterials is still in the progressing stage of technology.

With the intervening time, nanotechnology has been revolutionized by the use of 2D nanomaterials. 2D nanomaterials cover a number of important topics in basic and applied sciences making a novel class of materials with a promising future. 2D nanomaterials with high surface area, high electronic and excellent optical properties will be very useful in the near future for nano-electronic applications to be utilized in lithium-ion batteries, image sensors, biosensors, solar cells, supercapacitors, and catalysts. However, it is noteworthy that the cost of 2D nanomaterials is relatively higher, but as technologies will reach the maturing phase, the use of 2D nanomaterials will nurture, thus enabling the higher demand for targeted applications with lower prices that may start to establish themselves in the marketplace. It is anticipated that the contents summarized in this chapter can afford an important reference and guideline for further systematic studies on 2D nanomaterials.