**2. Transition metal dichalcogenides (TMDCs)**

TMDCs are denoted by the general formula of MX2 where M and X represent a transition metal (Mo, Nb, W, Hf, Ti, and so on), and a chalcogen (S, Te, and Se), respectively. In the periodic table, groups IV-X belong to the transition metals and they contain different number of d-electrons. Thus, the difference in the valance d-electrons of different transition metals gives rise to the different electronic properties such as metallic, semiconducting, and superconducting [33]. TMDCs exist in a layered form at the atomic level, containing one or few monolayers. **Figure 1(a)** shows a schematic depicting the layered structure of MoS2.

A TMDC can exist in various structural phases which are a result of the different co-ordinations of the transition metal atom. The most common structural phases in which the TMDCs crystallize are the trigonal prismatic (2H) and the octahedral (1T) phase. These crystal phases can also be seen in the terms of different stacking sequences of the atoms (as a representative result, **Figure 1(b)** shows crystal structure of MoS2). The three atomic planes i.e. chalcogen– metal–chalcogen form the individual layers of TMDCs. The 2H phase corresponds to an ABA stacking, whereas, the 1T phase is characterized by an ABC stacking order. For most of the bulk TMDCs (MoS2, MoSe2, MoTe2, WS2, WSe2, etc.), the 2H phase is thermodynamically more stable than the metastable 1T phase. Tungsten ditelluride (WTe2) shows an exception where the most stable phase is the orthorhombic (1Td phase) at room temperature [33].

#### **Figure 1.**

*Practical Applications of Laser Ablation*

ment of multifunctional PDs.

and from rigid to flexible electronic devices [29–31].

Regardless of the efforts of the researchers and scientists, some common challenges are still being faced related to fabrication and the performance of these TMDCs-based devices [18]. The challenges include growth of high-quality crystals, controlling the morphology and the thickness, scaling up the growth for industrial scale production, optimizing the device architectures, and so forth. To address these challenges, pulsed laser deposition (PLD) has emerged as a perfect tool for the synthesis of TMDCs. With the use of PLD, the actualization of high quality and wafer scale synthesis of TMDCs has become possible [32]. Eminently, PDs based on the PLD-synthesized TMDCs have exhibited competitive device performance

in photovoltaics [1, 2], military and defense technology [3], optical communication [4], remote sensing [3], biomedical imaging [5], environmental and ozone layer monitoring [4], and so on. Therefore, highly efficient photodetection has become very crucial for the industrial and scientific communities. With advancements in the matured technology of three-dimensional (3D) semiconductors [6–17] such as gallium nitride (GaN), zinc oxide (ZnO), indium gallium nitride (InGaN), indium nitride (InN), gallium oxide (Ga2O3), gallium arsenide (GaAs), silicon (Si), aluminum gallium nitride (AlGaN), germanium (Ge), mercury cadmium telluride (HgCdTe), gallium antimonide (GaSb), and so forth, highperformance PDs sensitive to wavelengths in the entire ultraviolet (UV)-far infrared (FIR) have been successfully fabricated. However, further advances in these PDs are hindered by the certain drawbacks encountered due to the intrinsic limitations in 3D semiconductors such as lower charge carrier mobilities, low light absorption properties, presence of dangling bonds at the interface, high fabrication costs involved, and so forth [18]. Thus, it is crucial to explore alternate materials, which can overcome the above-mentioned drawbacks for the develop-

The successful delamination of graphene in the revolutionary work by Geim and Novoselov in 2004 [19] ignited a plethora of research, in the field of two-dimensional (2D) layered materials and their heterostructures [20–22]. In the recent years, the amount of research focused on these layered materials has increased multifold. A layered material is nothing but an ultrathin phase of a material, scaled down to the level of atomic thickness, and is characterized by weak inter-layer van der Waals (vdW) forces and a strong intra-layer covalent interaction [23]. This makes these ultrathin materials possess electronic and optoelectronic properties such as band gap, mobility, etc., that are thickness-dependent [24], and thus, their novel chemical and physical characteristics pave a way towards the unexplored areas, both in the fields of fundamental research as well as engineering applications. In spite of possessing unparalleled electronic and optoelectronic properties such as high carrier mobility, dangling bonds-free surface, large current carrying capacities, excellent mechanical properties, the zero band gap or the gapless electronic structure of graphene [25] limits its use in realization of practical applications-based PDs, which demand switching behavior or in other words, a definite on/off state. This has led to the exploration of graphene alternatives with a substantial band gap, and researchers and scientists across the world have resurrected a class of conventional 2D materials known as the transition metal dichalcogenides (TMDCs), characterized by low-fabrication cost, chemical stability, earth-abundance and environment-friendly properties. Some of the well-studied TMDCs are molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum diselenide (MoSe2), molybdenum ditelluride (MoTe2), tungsten diselenide (WSe2), and so on [26–28]. As of now, semiconductors of the TMDC family have enabled tremendous accomplishments in the field of photodetection, such as from monofunctional to multifunctional PDs, from homogeneous to hybrid 2D semiconductors-based PDs,

**24**

*(a) Schematic showing a monolayer of a TMDC, where the atoms of transition metal are bonded through covalent bonds with the chalcogen atoms. These individual monolayers are stacked and held together by vdW forces to form the bulk structures. (b) Crystal structures of layered MoS2 with different stacking sequences as shown to form the two most common phases: trigonal prismatic (2H) and octahedral (1T). Figure is adapted from Ref. [23]. (c) Transformation of the band structure of 2H phase of MoS2 calculated by first principles from bulk to a single layer. Figure is adapted and reproduced with permission from Ref. [34].*

The assortment of chemical compositions as well as the different crystal structures of TMDCs results in varying band structure characters, which in turn lead to a wide range of electronic properties. In the thermodynamically most stable 2H phase, MoS2, WS2, MoSe2, and WSe2 show semiconducting behavior [33]. These semiconducting properties accentuated these TMDCs as potential 2D materials for next generation electronic devices. As a representative result, the basic characteristics of the band structure of MoS2 has been shown in **Figure 1(c)**. The transformation of the band structure as calculated from density functional theory (DFT) for 2H-MoS2 upon increasing its thickness from monolayer form to the bulk has been shown in **Figure 1(c)**. The positions of the conduction and valence band edges change with increasing the number of layers of MoS2, and the direct band gap in the monolayer form changes into an indirect band gap in the bulk material [33]. The calculated value of the band gap of the monolayer 2H-MoS2 is ~1.89 eV [35]. The experimentally observed value for the electronic band gap of 2H-MoS2 in its monolayer form is 2.15 eV [36]. Notably, the conduction band minimum and the valence band maximum are situated at the two inequivalent high-symmetry points, which represent the corners of the hexagonal Brillouin zone [33]. This attribute is common between monolayer 2H phase MoS2 (and the other group VI single-layer 2H phase TMDCs) and graphene and allows the observation of potential valleytronics applications.

Through the persistent efforts over the last decade, researchers have developed several techniques to produce ultrathin TMDCs. In principle, these techniques can be broadly classified into two categories. The first one is to produce TMDCs by thinning bulk crystals, which is called as a top-down technique. Different types of exfoliations (mechanical, chemical, etc.) fall under this category. Since different layers of 2D materials are held together by vdW interactions, therefore, the interlayer bonding is weak. Thus, under any external perturbations, bulk 2D materials are readily processed into their few-layered forms. The second category is to produce TMDCs through a bottom-up approach, where constituent atoms and molecules are assembled together to form continuous layers. These mainly include chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron sputtering, molecular beam epitaxy (MBE) and PLD.

As mentioned above, researchers have developed several methods to fabricate TMDCs. In spite of the substantial progress, none of the above techniques can meet the comprehensive demands for industrial-scale production, as in the terms of process simplicity, good scalability, excellent homogeneity and continuity, high quality of the products, high compositional and thickness control, low cost for mass production and higher safety. The techniques such as exfoliation, CVD and ALD suffer from a huge drawback that the growth process occurs in absence of high vacuum, which generally leads to unclean interfaces, and therefore, one has to compromise with the device performance. Moreover, exfoliation and CVD are further associated with low product yield as the films formed are non-continuous and in the range of several microns. Sputtering and MBE are much more sophisticated in terms of the interface quality due to the involvement of ultrahigh vacuum, however, sputtering is characterized by poor surface quality of the films whereas MBE poses drawbacks such as bulky and expensive setups, time-consuming growths, and limitations in terms of substrates. Such limitations further hinder the utilization of these growth methods for industrial scale fabrication of devices. Thus, research and development towards a potentially competent approach which can address the above issues is greatly required.

PLD is a synthesis technique where a high-power pulsed laser beam is focused on a target, which results in the vaporization of the material in the form of a plasma plume and this material gets deposited on a substrate in the form of a thin film.

**27**

**Figure 2.**

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures…*

A typical illustration of a PLD process is shown in **Figure 2**. Compared with the conventional methods discussed above, PLD exhibits the following advantages:

• PLD is the most versatile growth method where a focused and high-energy pulsed laser ionizes almost all types of materials, because of the generation of instantaneous and localized high temperatures up to tens of thousands of OC on the target's surface. Therefore, majority of the materials can be ablated to form a plasma, which carries out the deposition. In addition, PLD exhibits an excellent compatibility with different substrates which provides numerous

• PLD is highly scalable, because the plasma plume can be readily positioned and directed just by adjusting the external optical path of the laser beam. Consequently, PLD is very much suitable for the growth of wafer-scale and uniform TMDCs for practical industrial production. Serna *et al*. [38] have successfully fabricated continuous bilayers of MoS2 on sapphire with a diameter up to ~50.8 mm using PLD. Singh *et al*. [39] have also synthesized

• The substrate temperature required for the PLD growth of TMDCs is relatively low when compared with techniques like CVD and sputtering because the species (atoms, molecules, ions, etc.) ablated by the focused pulsed laser possess a very high energy, and therefore, can freely migrate on the substrate's surface. Therefore, direct growth of TMDCs on substrates that are intolerable towards high temperature can be achieved with PLD. For instance, Singh *et al*. [39] have successfully fabricated MoS2 on InN at a low substrate temperature of 450°C. It may be noted that low temperature deposition is required for InN

• PLD is a clean, highly efficient, safe, and highly controlled deposition technique where the product is highly continuous and uniform. Due to the ultrahigh vacuum growth conditions, the products of PLD are clean and contaminationfree. Furthermore, Siegel *et al*. [32] fabricated MoS2 on centimeter-scale

routes for the construction of heterojunctions-based devices.

centimeter-scale MoS2 thin films on various substrates.

as it dissociates above 500°C.

*DOI: http://dx.doi.org/10.5772/intechopen.94236*

*Schematic view of a typical PLD process. Adapted from Ref. [37].*

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures… DOI: http://dx.doi.org/10.5772/intechopen.94236*

#### **Figure 2.**

*Practical Applications of Laser Ablation*

ics applications.

The assortment of chemical compositions as well as the different crystal structures of TMDCs results in varying band structure characters, which in turn lead to a wide range of electronic properties. In the thermodynamically most stable 2H phase, MoS2, WS2, MoSe2, and WSe2 show semiconducting behavior [33]. These semiconducting properties accentuated these TMDCs as potential 2D materials for next generation electronic devices. As a representative result, the basic characteristics of the band structure of MoS2 has been shown in **Figure 1(c)**. The transformation of the band structure as calculated from density functional theory (DFT) for 2H-MoS2 upon increasing its thickness from monolayer form to the bulk has been shown in **Figure 1(c)**. The positions of the conduction and valence band edges change with increasing the number of layers of MoS2, and the direct band gap in the monolayer form changes into an indirect band gap in the bulk material [33]. The calculated value of the band gap of the monolayer 2H-MoS2 is ~1.89 eV [35]. The experimentally observed value for the electronic band gap of 2H-MoS2 in its monolayer form is 2.15 eV [36]. Notably, the conduction band minimum and the valence band maximum are situated at the two inequivalent high-symmetry points, which represent the corners of the hexagonal Brillouin zone [33]. This attribute is common between monolayer 2H phase MoS2 (and the other group VI single-layer 2H phase TMDCs) and graphene and allows the observation of potential valleytron-

Through the persistent efforts over the last decade, researchers have developed several techniques to produce ultrathin TMDCs. In principle, these techniques can be broadly classified into two categories. The first one is to produce TMDCs by thinning bulk crystals, which is called as a top-down technique. Different types of exfoliations (mechanical, chemical, etc.) fall under this category. Since different layers of 2D materials are held together by vdW interactions, therefore, the interlayer bonding is weak. Thus, under any external perturbations, bulk 2D materials are readily processed into their few-layered forms. The second category is to produce TMDCs through a bottom-up approach, where constituent atoms and molecules are assembled together to form continuous layers. These mainly include chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron

As mentioned above, researchers have developed several methods to fabricate TMDCs. In spite of the substantial progress, none of the above techniques can meet the comprehensive demands for industrial-scale production, as in the terms of process simplicity, good scalability, excellent homogeneity and continuity, high quality of the products, high compositional and thickness control, low cost for mass production and higher safety. The techniques such as exfoliation, CVD and ALD suffer from a huge drawback that the growth process occurs in absence of high vacuum, which generally leads to unclean interfaces, and therefore, one has to compromise with the device performance. Moreover, exfoliation and CVD are further associated with low product yield as the films formed are non-continuous and in the range of several microns. Sputtering and MBE are much more sophisticated in terms of the interface quality due to the involvement of ultrahigh vacuum, however, sputtering is characterized by poor surface quality of the films whereas MBE poses drawbacks such as bulky and expensive setups, time-consuming growths, and limitations in terms of substrates. Such limitations further hinder the utilization of these growth methods for industrial scale fabrication of devices. Thus, research and development towards a potentially

competent approach which can address the above issues is greatly required.

PLD is a synthesis technique where a high-power pulsed laser beam is focused on a target, which results in the vaporization of the material in the form of a plasma plume and this material gets deposited on a substrate in the form of a thin film.

sputtering, molecular beam epitaxy (MBE) and PLD.

**26**

*Schematic view of a typical PLD process. Adapted from Ref. [37].*

A typical illustration of a PLD process is shown in **Figure 2**. Compared with the conventional methods discussed above, PLD exhibits the following advantages:


sapphire substrates using PLD, varying the thickness from 60 monolayers down to a single monolayer, just by tuning the number of laser pulses.

In the past few years, TMDCs have elicited tremendous research interest, owing to their novel properties in the 2D form, which has triggered a spark in the growth of TMDCs using PLD [18]. In this part of the chapter, a few reports describing the chronological developments in the area of PLD grown TMDCs have been briefly discussed. One of the earliest works on PLD deposited MoS2 was reported by Zabinski *et al*. [40], where they have prepared PbO-MoS2 thin films for tribological applications. Other early reports on MoS2 thin films by PLD include growth of amorphous MoS2 by McDevitt *et al*. [41] and MoS2 coatings for friction-related studies by Mosleh *et al*. [42]. The major advancements in this area have been achieved in the past few years. In 2010, Fominski *et al*. [43] have experimentally studied the fabrication of MoSex thin films with varying compositions obtained by PLD in vacuum condition and in presence of different rarefied gases. They also developed a processbased mathematical model which played a dominant role on the chemical composition of these thin films. Loh *et al*. [44] in 2014 fabricated MoS2 on different metals such as Ag, Ni, Al, and Cu by PLD. In 2015, a significant step towards the fabrication of transfer-free TMDC-based PDs was demonstrated by Serrao *et al*. [45] in which MoS2 was directly deposited on substrates like sapphire, SiC-6H and GaN. The first significant work towards the wafer-scale growth of TMDCs by PLD was done by Siegel *et al*. [32] who reported growth of centimeter-scale MoS2 thin films of varying thickness (from monolayer to 60 monolayers). Growth of other members of the TMDC family such as WS2 was also investigated simultaneously. In 2015, Loh *et al*. [46] synthesized WS2 thin films on Ag substrates by PLD and reported the thickness dependent Raman and PL spectra. However, the obtained WS2 was having a mixed phase (1T and 2H). In subsequent works, Yao *et al*. [47] obtained the pure 2H phase WS2 directly by PLD on insulating SiO2/Si substrates. The growth of selenides via PLD usually suffers from a large number of Se vacancies. Therefore, a two-step growth method was adopted for production of MoSe2 [48]. This included deposition of MoO3 film via PLD, followed by its selenization. Later on, Mohammed *et al*. [49] achieved 1–8 monolayers of WSe2 via single-step deposition. This was achieved through a hybrid PLD cluster, where a tungsten target was ablated by the laser beam and selenium vapors were synchronously provided from an effusion cell by thermal evaporation. In 2018, a single-step PLD approach was used by Seo *et al*. [50] to deposit WSe2 on Al2O3 and SiO2/Si substrates by using a Se-rich target. Lately, Gao *et al*. [51] demonstrated a two-step synthesis route to fabricate 2D WTe2, which included PLD of amorphous WTe2 target followed by annealing treatment of the thin films in a Te atmosphere. These reports suggest that the technique of PLD can be suitably applied for the successful production of various TMDCs. In the next section, we will discuss in detail about the fundamentals associated with PLD.
