**1. Introduction**

Once a forbidden material, one atom thick material came into existence with the discovery of graphene in 2004 by Novoselov et al. [1]. Unsurprisingly due to the unique properties and momentous potential of 2D materials, the research world jumped into the foray. Interestingly the term "graphene" was coined in 1986 by Boehm and the electronic band structure of single layer graphite is studied since 1942 [2]. The surge of discovery didn't stop with graphene, it gathered pace, and "2D materials" like *h*-BN, transition metal dichalcogenides (TMDCs), and several other layered materials came into existence. This present work deals with one of the less explored TMDCs, i.e. PtSe2. Its unique and exciting properties are worth exploring. It has a wide range of applications like photodetectors, gas sensing, electronics,

piezoresistive sensors, and electrocatalysis [3–6]. Another interesting fact about PtSe2 is that it can be grown under relatively low temperatures when compared to other TMDCs via vapour phase synthesis process. This increases the choice of substrates. It has been reported to be grown at temperatures as low as 100°C [7]. Although PtSe2 emerged more than a century ago, its layer dependent properties and their respective applications are hardly explored. This chapter gives an outline of Raman spectroscopic studies of PtSe2 of different thicknesses, both at room temperature and low temperature. It also deals with the task of using Raman spectroscopy to measure in-plane thermal conductivity. Theoretical calculation of electronic band structure and density of states of the monolayer, bilayer, tri layer, and bulk PtSe2 has also been incorporated in this chapter.

The 1T phase of PtSe2 belongs to space group P m1 and this phase is more stable when compared to 1H phase [8]. The Bravais lattice of 1T phase monolayer PtSe2 is hexagonal and has D3d point group symmetry. The monolayer is made up of three atomic sub layers, Pt layer is being sandwiched between two Se layers. The reported lattice vectors are [9]. **Figure 1** shows the structure of PtSe2, i.e. top view (a) and side view (b). The calculated lattice constant of the primitive unit cell of monolayer PtSe2 is 3.70 Å which matches with the measured value of the same by STEM (scanning tunneling electron microscope) measurement [8, 10]. The primitive unit cell contains three atoms. The Pt-Se bond length for 1T phase is 2.52 Å and the distance between the top and bottom Se sub layers is 2.68 Å [10].

In monolayer 1T-PtSe2 phase the bonding is completely covalent in nature with no net transfer of charge between the bonded atoms. The work function of single layer PtSe2 is 5.36 eV and the values for other dichalcogenides like MoSe2 and WSe2 are 4.57 and 4.21 eV [8, 11]. The monolayer behaves as a semiconductor whereas the bulk behaves as a semi-metal [12, 13]. The phonon spectrum of mono layer 1T-PtSe2 consists of nine phonon modes out of which six are optical and three are acoustic. **Figure 2a** shows the phonon modes in 1L1 T-PtSe2. The six optical modes can be decomposed into - Γ = 2*E*g + 2*E*u + *A*1g + *A*2u. The in-plane modes (169 cm−1 for *E*g and 218 cm−1 for *E*u) are doubly degenerate and out of plane modes (200 cm−1 for *A*1g and 223 cm−1 for *A*2u) are singly degenerate, the (*A*1g + *E*g) modes are Raman active, 2*A*u is infrared active and 2*E*u is both infrared and Raman active. Out of these four modes, only two of them (*E*g mode at 169 cm−1 and *A*1g at 200 cm−1) are prominent. **Figure 2b** shows their calculated relative intensities along with the modes of vibration [8]. The strong covalent

#### **Figure 1.**

*(a) Top view and (b) side view of 1T phase PtSe2. Grey atoms represent Pt and green atoms represent Se, respectively [8].*

*Thickness Dependent Spectroscopic Studies in 2D PtSe2 DOI: http://dx.doi.org/10.5772/intechopen.103101*

#### **Figure 2.**

*(a) The phonon band diagram and (b) normalized Raman intensity of 1-layer 1T-PtSe2 [8]. (Grey atoms – Pt and green atom – Se).*

#### **Figure 3.**

*Raman spectrum of bilayer and multilayer (~5 nm thick) CVD grown PtSe2 at room temperature measured using laser wavelength of 532 nm [14].*

nature of the Pt-Se bond makes the in plane vibrational modes more intense when compared to out of plane modes.

**Figure 3** shows the Raman spectra of both bilayer and multilayer (~5 nm thick) CVD grown PtSe2 [14]. In bilayer PtSe2, two prominent peaks are observed which are centred at 179 and 207 cm−1. These peaks are associated with first order phonon emission of in-plane and out of plane vibrational modes i.e. *E*g and *A*1g modes. Additionally, another less prominent peak is recorded at 235 cm−1. This low intensity peak arises due to the longitudinal optical mode and can be separated into two vibrations. These two vibrations correspond to first order two phonon emission for out of plane (*A*2u) and in plane (*E*u) vibrations of Pt and Se atoms [14]. O'Brien et al. [15], reported the above mentioned peaks to be centred at 175 cm−1 (*E*g), 205 cm−1 (*A*1g), and 230 cm−1 (LO), and they also showed that these modes have approximately constant peak position for laser excitation of 488, 532, and 633 nm. The differences between theoretically derived (previously discussed) phonon modes and experimentally recorded ones can be attributed to the fact that PtSe2 layers are not pristine with definite thickness [15].
