**2. Structural and electronic properties of 2D NTMDs**

The atomic coordination of monolayer TMD usually is either trigonal prismatic phase (2H or D3h) or octahedral phase (1 T or D3d), as shown in schematics in **Figure 1a** and **b** [22, 29, 32]. In 2H phase, the d orbitals in transition metal centers split into three degenerated d orbitals ( <sup>2</sup> <sup>z</sup> <sup>d</sup> , <sup>−</sup> *y xy* 2 2 x , <sup>d</sup> and d*xz yz* , ) and there is usually an energy bandgap (~1 eV in TMDs) between the first two degenerated d orbitals. While in 1 T phase, the centers of transition metal have two degenerated d orbitals ( *z y* <sup>−</sup> 22 2 ,x d and d*xz yz xy* , , ) [22, 29, 32]. Therefore, the thermodynamically favored phase is highly influenced by d electrons count in the transition metals. For NTMDs, the noble metal atoms have abundant d electronics and the d<sup>2</sup> sp3 hybridization is preferred, which lead to the full-filled d-bands. Most NTMDs have thermo-dynamically favored 1 T phase, such as PtSe2, PtS2, PtTe2 and PdTe2 (See **Figure 1c**) [30]. The strong interlayer hybridization of adjacent chalcogen atoms makes the widely tunable electronic energy band structure with the layer numbers. Here we use PtS2 and PtSe2 as examples. Both of them are 1 T favored phase, where the bandgap is about 1.17 and 1.6 eV in monolayer PtSe2 and PtS2, respectively (**Figure 1e** and **f**) [21]. With the increase of stacked layers, the interlayer hybridization would be stronger, with lead to the rapidly decrease of energy gap. According to theoretical calculations, the energy gap in bi-layer PtSe2 is only 0.3 eV, while the stacked layered increase beyond 4 layers, the energy level of valence band

**133**

0.25 eV from 1.6 eV.

**Figure 1.**

*permission [26].*

**3. Optical properties of 2D NTMDs**

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector*

maximum (VBM) will exceed that of conduction band minimum (CBM) and PtSe2 undergoes a transition from semiconductor to metallic state (**Figure 1h**) [31]. Similarly, as shown in **Figure 1i**, the energy bandgap in bulk PtS2 decreases to

*Crystal and electronic structure of NTMDs. (a) and (b) schematic images of 2H and 1 T lattice phase in TMDs, reproduced with permission [29]. (c) Thermodynamically favored 1 T-phase structural schematic of PtSe2, reproduced with permission [30]. (d) Puckered pentagonal structure of PdSe2, reproduced with permission [26]. (e) Energy band structure of monolayer PtSe2, reproduced with permission [23]. (f) Energy band structure of monolayer PtS2, where bands mVB-2 were highlighted spanning the Brillouin zone by black dots. (g) Calculated electronic band structures of monolayer PdSe2 by the optPBE method. (h)-(j) evolution of energy bandgap as a function of the number of layers of PtSe2(h), PtS2(i) and PdSe2(j). (h) Is reproduced with permission [31]. (f) and (i) are reproduced with permission [21]. (g) and (j) are reproduced with* 

Apart from conventional TMDs materials with hexagonal structures, PdS2 and PdSe2 consist of pentagonal rings with the puckered vertical conformation (**Figure 1d**) [26]. In each layer, a Pd atom binds to four chalcogen atoms other than six chalcogen atoms, while every two neighbor chalcogen atoms bind each other with a covalent bond. The unique pentagonal structure not only provides the materials with anisotropic properties, but also can realize the transition of topological quantum phase and the spin-orbit coupling enhancement. In 2017, Akinola O., et al. experimentally and theoretically prove that monolayer PtSe2 has 1.3 eV indirect band gap and semi-metal state in the bulk (**Figure 1g** and **j**) [26].

The widely tunable electronic energy gap of NTMDs make them layerdependent optical absorption [31]. As shown in **Figure 2a**, PtSe2 samples with thickness from 2.2 nm to 7.8 nm show broadband light absorption from 450 nm to over 3000 nm. The absorption peaks have significant red-shift with the increase

*DOI: http://dx.doi.org/10.5772/intechopen.95883*

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector DOI: http://dx.doi.org/10.5772/intechopen.95883*

#### **Figure 1.**

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

Two-dimensional (2D) materials have attracted tremendous attention in the past few decades [6–12]. Among them, 2D Transition-Metal Dichalcogenides (TMDs) are considered to be promising for next-generation optoelectronics due to the strong light-matter interaction, weak interlayer van der Waals (vdW) interaction, flexible characteristics and the ease of integration with current silicon-based optical electronics [13–17]. Group-10 noble TMDs (NTMDs) are outstanding representatives in the TMDs family [18–20]. The reintroduced new materials are generalized formulated by Group-10 noble elements (Pt, Pd, and so on*.*) and chalcogens (S, Se, or Te). Unlike traditional TMDs, the d-electrons in NTMDs are fully occupied their d-orbitals resulting in the highly hybridized Pz orbits and strong interlayer interactions [21, 22]. Therefore, NTMDs exhibit relatively small and widely tunable bandgaps compared with traditional TMDs (such as MoS2 and WS2). For example, PtS2 shows a layer-dependent bandgap from 1.6 to 0.25 eV [21], while PtSe2 changes from a typical semiconductor state (1.2 eV in 1 L PtSe2) to semi-metal state when the thickness increases to over 5 layers [23]. Combining with the high mobility

V−1S−1, larger than most other TMDs and comparable for that of BP) and

environmental stability, NTMDs has great potential in photodetectors applications [21, 23–25]. Moreover, the unique puckered pentagonal structure of PdS2 and PdSe2 inherently provides them with anisotropic properties [26–28] and may promote the

In this chapter, we first discuss the structural, electronic and optical properties of NTMDs. Then we focus on the NTMDs based photodetectors. Waferscale NTMDs films with high-quality and large-scale monocrystalline NTMDs nanosheets have been fabricated, which are appropriate for optoelectronic applications. NTMDs and their heterostructure based photodetectors show many advantages such as high-performance, ultrawide spectra detection, long-term environment stability, and anisotropic characteristics. NTMDs have great potential for large-scale imaging and flexible devices, which could be the next-generation

The atomic coordination of monolayer TMD usually is either trigonal prismatic

sp3

hybrid-

phase (2H or D3h) or octahedral phase (1 T or D3d), as shown in schematics in **Figure 1a** and **b** [22, 29, 32]. In 2H phase, the d orbitals in transition metal centers split into three degenerated d orbitals ( <sup>2</sup> <sup>z</sup> <sup>d</sup> , <sup>−</sup> *y xy* 2 2 x , <sup>d</sup> and d*xz yz* , ) and there is usually an energy bandgap (~1 eV in TMDs) between the first two degenerated d orbitals. While in 1 T phase, the centers of transition metal have two degenerated d orbitals ( *z y* <sup>−</sup> 22 2 ,x d and d*xz yz xy* , , ) [22, 29, 32]. Therefore, the thermodynamically favored phase is highly influenced by d electrons count in the transition metals. For NTMDs, the noble metal atoms have abundant d electronics and the d<sup>2</sup>

ization is preferred, which lead to the full-filled d-bands. Most NTMDs have thermo-dynamically favored 1 T phase, such as PtSe2, PtS2, PtTe2 and PdTe2 (See **Figure 1c**) [30]. The strong interlayer hybridization of adjacent chalcogen atoms makes the widely tunable electronic energy band structure with the layer numbers. Here we use PtS2 and PtSe2 as examples. Both of them are 1 T favored phase, where the bandgap is about 1.17 and 1.6 eV in monolayer PtSe2 and PtS2, respectively (**Figure 1e** and **f**) [21]. With the increase of stacked layers, the interlayer hybridization would be stronger, with lead to the rapidly decrease of energy gap. According to theoretical calculations, the energy gap in bi-layer PtSe2 is only 0.3 eV, while the

stacked layered increase beyond 4 layers, the energy level of valence band

**132**

(>1000 cm2

development of polarized photodetectors.

**2. Structural and electronic properties of 2D NTMDs**

optoelectronic core materials.

*Crystal and electronic structure of NTMDs. (a) and (b) schematic images of 2H and 1 T lattice phase in TMDs, reproduced with permission [29]. (c) Thermodynamically favored 1 T-phase structural schematic of PtSe2, reproduced with permission [30]. (d) Puckered pentagonal structure of PdSe2, reproduced with permission [26]. (e) Energy band structure of monolayer PtSe2, reproduced with permission [23]. (f) Energy band structure of monolayer PtS2, where bands mVB-2 were highlighted spanning the Brillouin zone by black dots. (g) Calculated electronic band structures of monolayer PdSe2 by the optPBE method. (h)-(j) evolution of energy bandgap as a function of the number of layers of PtSe2(h), PtS2(i) and PdSe2(j). (h) Is reproduced with permission [31]. (f) and (i) are reproduced with permission [21]. (g) and (j) are reproduced with permission [26].*

maximum (VBM) will exceed that of conduction band minimum (CBM) and PtSe2 undergoes a transition from semiconductor to metallic state (**Figure 1h**) [31]. Similarly, as shown in **Figure 1i**, the energy bandgap in bulk PtS2 decreases to 0.25 eV from 1.6 eV.

Apart from conventional TMDs materials with hexagonal structures, PdS2 and PdSe2 consist of pentagonal rings with the puckered vertical conformation (**Figure 1d**) [26]. In each layer, a Pd atom binds to four chalcogen atoms other than six chalcogen atoms, while every two neighbor chalcogen atoms bind each other with a covalent bond. The unique pentagonal structure not only provides the materials with anisotropic properties, but also can realize the transition of topological quantum phase and the spin-orbit coupling enhancement. In 2017, Akinola O., et al. experimentally and theoretically prove that monolayer PtSe2 has 1.3 eV indirect band gap and semi-metal state in the bulk (**Figure 1g** and **j**) [26].

#### **3. Optical properties of 2D NTMDs**

The widely tunable electronic energy gap of NTMDs make them layerdependent optical absorption [31]. As shown in **Figure 2a**, PtSe2 samples with thickness from 2.2 nm to 7.8 nm show broadband light absorption from 450 nm to over 3000 nm. The absorption peaks have significant red-shift with the increase

#### **Figure 2.**

*Optical properties of NTMDs. (a)-(b) Vis-near-IR and mid-IR absorption spectra of PtSe2 with different thickness. The substrates are sapphires and the (a) inset is optical absorption spectrum of 5 nm thick Au film as reference. (c) Reflective intensity of RGB channels as the function of rotational incident angle, which reflect the in-plane isotropic absorption of PtSe2. (a)-(c) are reproduced with permission [33]. (d)and (e) calculated optical conductivity spectra of 1 L and bulk PdSe2, reproduced with permission [34]. (f) Integrated SHG intensity diagram with different rotation angle of line-polarized laser, reproduced with permission [35].*

of thickness, which is originates from the narrower energy gap in thicker samples. In particular, the semi-metal nature in thick PtSe2 samples allows them absorb mid-NIR and even far-NIR light. In **Figure 2b**, all of these samples have broadband absorption in the range from 2 to 5 μm, which is different from tradition TMDs materials. The optical polarization properties of PtSe2 were studied by polarized light imaging experiments. The optical responses of 2D PtSe2 film almost unchanged under the incident channel with different rotation angle, which indicate the in-plane isotropic absorption of PtSe2 (**Figure 2c**).

On contrary, due to the unique orthorhombic pentagonal structure, PdSe2 shows anisotropic optical response in the van der Waals plane [34]. From the calculated optical conductivity spectra in **Figure 2d** and **e**, the cut-off energy in bulk PdSe2 is lower than that in 1 L PdSe2, and the conductivity curves in xx and yy direction in both bulk and 2D PdSe2 perform very different characteristics. The anisotropic phenomenon appears at ~1.5 and 1.25 eV in bulk and 2D structure, respectively. The large anisotropy also be predicted at ~2 eV in monolayer PdSe2. Second harmonic generation (SHG) polarization diagram is also performed for observing the anisotropic properties [35]. When the polarization direction of incident light and the crystal orientation are parallel (position of 0° and 180° in **Figure 2f**), the intensity achieves the maximum, while at the position of 0° and 180°, the SHG signal shows significantly decrease.

#### **4. Synthesize of 2D NTMDs**

In order to realizing the practical applications of the new kind of TMDs materials, the effective synthesis methods are essential to prepare particular samples with high crystallinity quality, desirable thickness and large lateral size. Up to now, various of synthesis strategies have been conducted to a variety of high-quality NTMDs. Here we do a general review on the different fabrication methods for NTMDs. Chemical vapor transport (CVT) and chemical vapor deposition (CVD) techniques

**135**

**Figure 3.**

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector*

are most two important methods for NTMDs which are applied to the following

CVT method is a traditional crystal growth method, which is recently reintroduced for the direct synthesis of TMDs with high crystal quality [36–38]. The synthesis setup is as shown in **Figure 3a** [39]. Pt and Se powders with strict ratio are loaded in the quartz ampoule. After the vapor reactions with the help of a gaseous reactant under high temperature and vacuum, PtSe2 crystals are formed and deposited elsewhere. By carefully adjusting the amounts of reactants and transport, Hu et al. successfully obtained triangular-shaped PtSe2 flakes with 10–50 μm and good controllability [41]. From **Figure 3d**, the optical images exhibit that PtSe2 nanoflakes have controlled layer numbers from 1-layer (1 L) to 20 layers (~20 L) and the atomic force microscope (AFM) images in insets provide the thickness information. The as-grown nanoplates with monocrystalline structure, controllable thickness and large lateral size are very suitable for electronic and photonic devices. Due to the ease of growing bulk crystals by CVT, people also use this method to grow high quality single-crystalline bulk NTMDs and obtain one- to few-layer 2D flakes by

CVD is a very common synthesis method in which a large number of 2D materials with scalable size, controllable thickness and high-quality crystal structure have been prepared such as graphene, TMDs, Xene, MXene, boron nitride and so on [44–46]. Recently, the CVD method also be adopted for large-scale NTMDs fabrication. **Figure 3b** shows a CVD selenization method for scalable PtSe2 films. The Pt film as seed were deposited on the substrate (usually the SiO2 or Si wafer) at first and placed in the center of CVD furnace. The Se powder is at the upstream

*Materials fabrication for large-scale films and monocrystalline nanosheets. (a) Schematic of CVT method for PtSe2 with controllable thickness, reproduced with permission [39]. (b) Schematic of CVD selenization method for scalable PtSe2 films, reproduced with permission [40]. (c) Schematic of CVD method for the controlled synthesis of NTMDs nanosheets, reproduced with permission [39]. (d) Optical images of PVT-grown PtSe2 flakes with 10–50* μ*m and controlled layer numbers from 1 L to ~20 L, reproduced with permission [41]. (e) Photographs of CVD-grown 2D PtSe2 polycrystal films from 0.75 to 10 nm, reproduced with permission [42]. (f) Material characterizations for PtSe2 single crystal nanosheets by CVD method, including HAADF-STEM,* 

*EDS, Raman, HRTEM and SEAD techniques, reproduced with permission [43].*

*DOI: http://dx.doi.org/10.5772/intechopen.95883*

peeled from the bulk NTMDs crystal.

photodetector applications.

#### *Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector DOI: http://dx.doi.org/10.5772/intechopen.95883*

are most two important methods for NTMDs which are applied to the following photodetector applications.

CVT method is a traditional crystal growth method, which is recently reintroduced for the direct synthesis of TMDs with high crystal quality [36–38]. The synthesis setup is as shown in **Figure 3a** [39]. Pt and Se powders with strict ratio are loaded in the quartz ampoule. After the vapor reactions with the help of a gaseous reactant under high temperature and vacuum, PtSe2 crystals are formed and deposited elsewhere. By carefully adjusting the amounts of reactants and transport, Hu et al. successfully obtained triangular-shaped PtSe2 flakes with 10–50 μm and good controllability [41]. From **Figure 3d**, the optical images exhibit that PtSe2 nanoflakes have controlled layer numbers from 1-layer (1 L) to 20 layers (~20 L) and the atomic force microscope (AFM) images in insets provide the thickness information. The as-grown nanoplates with monocrystalline structure, controllable thickness and large lateral size are very suitable for electronic and photonic devices. Due to the ease of growing bulk crystals by CVT, people also use this method to grow high quality single-crystalline bulk NTMDs and obtain one- to few-layer 2D flakes by peeled from the bulk NTMDs crystal.

CVD is a very common synthesis method in which a large number of 2D materials with scalable size, controllable thickness and high-quality crystal structure have been prepared such as graphene, TMDs, Xene, MXene, boron nitride and so on [44–46]. Recently, the CVD method also be adopted for large-scale NTMDs fabrication. **Figure 3b** shows a CVD selenization method for scalable PtSe2 films. The Pt film as seed were deposited on the substrate (usually the SiO2 or Si wafer) at first and placed in the center of CVD furnace. The Se powder is at the upstream

#### **Figure 3.**

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

of thickness, which is originates from the narrower energy gap in thicker samples. In particular, the semi-metal nature in thick PtSe2 samples allows them absorb mid-NIR and even far-NIR light. In **Figure 2b**, all of these samples have broadband absorption in the range from 2 to 5 μm, which is different from tradition TMDs materials. The optical polarization properties of PtSe2 were studied by polarized light imaging experiments. The optical responses of 2D PtSe2 film almost unchanged under the incident channel with different rotation angle, which indicate

*Optical properties of NTMDs. (a)-(b) Vis-near-IR and mid-IR absorption spectra of PtSe2 with different thickness. The substrates are sapphires and the (a) inset is optical absorption spectrum of 5 nm thick Au film as reference. (c) Reflective intensity of RGB channels as the function of rotational incident angle, which reflect the in-plane isotropic absorption of PtSe2. (a)-(c) are reproduced with permission [33]. (d)and (e) calculated optical conductivity spectra of 1 L and bulk PdSe2, reproduced with permission [34]. (f) Integrated SHG intensity diagram with different rotation angle of line-polarized laser, reproduced with permission [35].*

On contrary, due to the unique orthorhombic pentagonal structure, PdSe2 shows anisotropic optical response in the van der Waals plane [34]. From the calculated optical conductivity spectra in **Figure 2d** and **e**, the cut-off energy in bulk PdSe2 is lower than that in 1 L PdSe2, and the conductivity curves in xx and yy direction in both bulk and 2D PdSe2 perform very different characteristics. The anisotropic phenomenon appears at ~1.5 and 1.25 eV in bulk and 2D structure, respectively. The large anisotropy also be predicted at ~2 eV in monolayer PdSe2. Second harmonic generation (SHG) polarization diagram is also performed for observing the anisotropic properties [35]. When the polarization direction of incident light and the crystal orientation are parallel (position of 0° and 180° in **Figure 2f**), the intensity achieves the maximum, while at

In order to realizing the practical applications of the new kind of TMDs materials, the effective synthesis methods are essential to prepare particular samples with high crystallinity quality, desirable thickness and large lateral size. Up to now, various of synthesis strategies have been conducted to a variety of high-quality NTMDs. Here we do a general review on the different fabrication methods for NTMDs. Chemical vapor transport (CVT) and chemical vapor deposition (CVD) techniques

the position of 0° and 180°, the SHG signal shows significantly decrease.

the in-plane isotropic absorption of PtSe2 (**Figure 2c**).

**4. Synthesize of 2D NTMDs**

**134**

**Figure 2.**

*Materials fabrication for large-scale films and monocrystalline nanosheets. (a) Schematic of CVT method for PtSe2 with controllable thickness, reproduced with permission [39]. (b) Schematic of CVD selenization method for scalable PtSe2 films, reproduced with permission [40]. (c) Schematic of CVD method for the controlled synthesis of NTMDs nanosheets, reproduced with permission [39]. (d) Optical images of PVT-grown PtSe2 flakes with 10–50* μ*m and controlled layer numbers from 1 L to ~20 L, reproduced with permission [41]. (e) Photographs of CVD-grown 2D PtSe2 polycrystal films from 0.75 to 10 nm, reproduced with permission [42]. (f) Material characterizations for PtSe2 single crystal nanosheets by CVD method, including HAADF-STEM, EDS, Raman, HRTEM and SEAD techniques, reproduced with permission [43].*

side. Then the direct selenization of the Pt film happens under high temperature, low pressure and argon gas protection. In 2015, Wang et al. firstly synthesized monolayer PtSe2 nanosheets [40]. Then Han et al. obtained large area PtSe2 film (> a few cm<sup>2</sup> ) with controllable thickness [42]. **Figure 3e** shows the photographs of as-grown 2D PtSe2 polycrystal films from 0.75 to 10 nm (corresponding to the layer numbers from 1 L to ~15 L). In 2018, Yuan et al. successfully fabricated PtSe2-PtS2 heterostructure film with wafer-scale and successfully achieved the wafer-scale photodetector application [47]. Besides, CVD method can also synthesize high-quality 2D NTMDs nanocrystals. **Figure 3c** exhibits a schematic of growing 2D nanosheets and through the method, Ma et al. successfully fabricated 2D PtTe2 nanoplates with tunable thickness and a large lateral size up to 80 μm [43]. From **Figure 3f**, the high-angle annular darkfield scanning-TEM (HAADF-STEM) image as well as the EDS mapping analysis shows the well-faceted triangular geometry and the uniformly spatial distribution of Pt and Te elements. The Raman spectrum and High-resolution TEM (HRTEM) furtherly show the high quality of nanosheets and the 6-fold symmetry SEAD pattern shows the hexagonal crystal structure. Type-II Dirac fermions are observed in the highquality nanocrystal platform. Another advantage of the grown method is that 2D materials can be grown on arbitrary substrates, because both the pre-deposition and post-selenization process do not have strict requirements to the substrate. Till now, 2D NTMDs have been fabricated on different substrates including Si, SiO2, Sapphire, gallium nitride (GaN), fused quartz, and flexible polyimide.

There are some other synthesize ways for atomic TMDs. Mechanical exfoliation (ME) is one of the most extensively adopted approaches for 2D nanoflakes from their bulk counterparts [13]. Therefore, the as-prepared 2D flakes can maintain the intrinsic structure. Nowadays, most of mechanically exfoliated NTMDs thin flakes are from bulk crystals grown by CVT [48] and self-flux method [26, 49]. These typical nanosheets show the extraordinary electronic properties, but their small lateral size and uncontrollability during the fabrication process limit their application potential in practical devices. Molecular beam epitaxy (MBE) has also been applied for 2D NTMDs, including PtSe2 [50], PdTe2 [51] and PdSe2 [52], which shows the merits of large-size monocrystalline, and controllable thickness on different substrates. For example, the high-quality PtSe2 atomic film was epitaxial grown on bi-layer graphene/6H-SiC substrate through MBE method [50]. The as-grown film had controllable thickness from single-layer to over 22 layers, which shows extraordinary thickness-dependent electronic properties and tunable bandgaps.
