6-1 Raman scattering spectra

Figure 6.1 shows room-temperature Raman scattering spectrum of a monolayer WS2 thin film

on sapphire substrate excited by a 532-nm laser line. The spectrum comprises two main first-order

Raman phonon modes and many weak phonon modes. We fitted these phonon peaks by using a

standard Lorentzian model. Our fitting curves are shown in Fig. 6.2. Table 6.1 contains a list of

fitting parameters. According to factor group analysis, the crystal structure of 2H-WX₂ (X = S, Se)

has hexagonal structure (space group P63/mmc) containing two formula per primitive cell. Figure

6.3a is a schematic representation of the crystal structure of WX2. Single layer is composed of two

hexagonal planes of X atoms and an intercalated hexagonal plane of W atoms bound with X atoms in

a trigonal prismatic arrangement. For bulk WX2, the repeat distance along the c-axis includes two of

single layers (Fig. 6.3b). Single layers are separated by weak Van der Waals forces between the X

atom layers of adjacent sheets. There are 18 lattice dynamical modes at the center of the Brillouin

zone (Γ-point). The decomposition into irreducible representations is as follows [133-136] : Γ_{bulk all,} =A_1g +2A_2u +B_1u +B₂¹_g +B₂²_g+2E_1u +E_1g+E_2u+E₂¹_g +E₂²_g.

The transformation properties of these representations are listed in Table 6.2. Acoustical modes and infrared-active optical modes must transform as either T_x, T_y, or T_z and must be antisymmetric

under inversion. This implies that the three acoustical vibrations consist of one mode of A_2u

symmetry and two degenerate modes of E_1u symmetry. Similarly, two infrared active modes are

expected, one mode of A_2u symmetry and one degenerate modes of E_1u symmetry. The

correlation among representations is presented in the correlation chart of Table 6.3. The tungsten atoms in W(S, Se)2 occupy sites of D_3h symmetry, whereas the sulfur (selenium) atoms are at sites of C_3v symmetry [137,138]. All lattice dynamical modes minus acoustical modes to get Γbulk optical_, .

The decomposition into irreducible representations is as follows:

1 2 1 2

P63/mmc to P6m2. Therefore, the lattice dynamical modes must be changed accordingly. Monolayer

WX2 contains one formula per primitive cell. There is no longer a c-axis and the rigid layer shear mode E_{2 g}² is absent in monolayers. Γbulk optical_, minus E_{2 g}² mode and Γ_G ( A_2u +B₂¹_g +B₂²_g ),

which define that the direction of vibration is c-axis and the W and X atoms are involved, to get

, monolayer optical

Γ . Therefore, monolayer WX₂ has 9 lattice dynamical modes at the center of the

Brillouin zone. The decomposition into irreducible representations is as follows:

Γbulk optical_{, ,} −E₂²_g − Γ = Γ_G monolayer optical =A_1g +B_1u+E_1u+E_1g+E_2u+E₂¹_g,

In a monolayer, the lattice vibration modes of E_1u and E_2u are the same as E¹_{2 g}and E_1g

vibrational modes (Fig. 6.4) [134]. Consequently, the Γ-point normal modes can be decomposed as:

Γmonolayer optical_, = A_1g+B_1u+E_1g +E¹_2g.

These modes are classified as Raman-active (A_1g +E_1g+E₂¹_g) and inactive (B_1u).

The peak at approximately 148.2 cm^-1 corresponds to LA

( )

M −E2²_g

( )

Γ mode. The peak at approximately 175.6 cm^-1 is related to ^LA

( )

^M mode. The peak at approximately 192.6 cm^-1 is

connected with ^LA

( )

^K mode. The peak at approximately 200.6 cm^-1 corresponds to

( )

M 2²_g

( )

LA +E Γ mode. The peak at approximately 214.8 cm^-1 is related to LA

( )

K +E2²_g

( )

Γ mode. The peak at approximately 231.2 cm^-1 is connected with A1_g

( )

M −LA

( )

M mode. The peak at approximately 266.8 cm^-1 corresponds to 2LA

( )

M −3E2²_g

( )

Γ mode. The peak at approximately 297.9 cm^-1 is related to 2LA

( )

M −2E2²_g

( )

Γ mode. The peak at approximately 326.0 cm^-1 is

connected with 2LA

( )

M −E2²_g

( )

Γ mode. The peak at approximately 345.4 cm^-1 corresponds to

( )

1

2_g M

E mode. The peak at approximately 353.0 cm^-1 is related to ^2LA

( )

^M mode. The peak at approximately 357.3 cm^-1 is connected with E2 g¹

( )

Γ mode. The peak at approximately 417.8 cm^-1 corresponds to A1g

( )

Γ mode. The peak at approximately 578.9 cm^-1 is related to ^3LA

( )

^{K mode.}

The peak at approximately 586.0 cm^-1 is connected with A1_g

( )

M +LA

( )

M mode. The peak at approximately 705.9 cm^-1 is related to ^4LA

( )

^M mode [65,140,141]. The first-order phonon modes include ^LA

( )

^{M , LA}

( )

^{K ,} E¹2_g

( )

M , E¹2 g

( )

Γ and A1g

( )

Γ . ^2LA

( )

^M is the second-order phonon mode. Here we focus on three phonon modes, A1g

( )

Γ , E¹2 g

( )

Γ , and ^2LA

( )

^{M . The} A1g

( )

Γ mode is an out-of-plane vibration involving only the sulfur atoms. The E2 g¹

( )

Γ mode is an in-plane vibration involving the displacement of tungsten and sulfur atoms. Our Raman spectrum of a

CVD-grown monolayer WS₂ thin film is in good agreement with that reported earlier by

Thripuranthaka M. et al. [65] They measured the Raman spectrum of single-layer WS2 film prepared

by mechanical exfoliation.

Figure 6.5 shows the difference of the peak frequency in these two phonon modes. It is

approximately 60.5 cm^-1, indicating a single-layer signature [69,142]. The stronger intensity of

( )

2LA M mode, which is approximately four times the intensity of A1g

( )

Γ mode, is due to double resonance scattering. The ^2LA

( )

^M mode overlaps with the E2 g¹

( )

Γ mode. The double resonant scattering process involves two phonons with equal and opposite momentum and an intermediate

excited electronic state that resonates with the electronic band structure. For a second-order Raman

mode, process to satisfy the requirements of double resonance is that the optical excitation energy

must match a vertical electronic transition. The electron in the conduction band then experiences two electron-phonon scattering events, the first scattering event involving a phonon with momentum + q

and the second involving a phonon with momentum −q. The momentum dependence of the

electronic structure and phonon dispersion must combine to produce sharp peaks in momentum

space for the double resonant scattering process, in order to produce a sharp Raman feature

[140,141].

Figure 6.6 displays room-temperature Raman scattering spectrum of a monolayer WS2 thin film on sapphire substrate excited by a 785-nm laser line. Only E¹2 g

( )

Γ phonon mode (≈ 358.2 cm^-1) is visible in the 785-nm of excitation (Fig. 6.5). The A1g

( )

Γ mode is obscured by the sapphire

substrate.

Figure 6.8 shows room-temperature Raman scattering spectrum of a monolayer WSe2 thin film

on sapphire substrate excited by a 532-nm laser line. We fitted the phonon peaks by using a standard

Lorentzian model. Our fitting curves are shown in Fig. 6.9. Table 6.4 contains a list of fitting

parameters. The spectrum comprises 6 phonon modes. The peak at approximately 137.2 cm^-1 is

related to A1_g

( )

Γ −LA

( )

M mode. The peak at approximately 250.0 cm^-1 is connected with

( )

1

E2 g Γ mode. The peak at approximately 261.1 cm^-1 is related to A1g

( )

Γ mode. The peak at approximately 359.8 cm^-1 corresponds to 2E1¹_g

( )

Γ mode. The peak at approximately 374.0 cm^-1 is

related to A1_g

( )

Γ +LA

( )

M mode. The peak at approximately 359.9 cm^-1 is connected with

( ) ( )

2A1_g Γ −LA M mode [66,143]. Our Raman spectrum of a CVD-grown monolayer WSe2 thin film is in good agreement with the literature reported by E. D. Corro et al. [66] They measured the

Raman spectra of single-layer WSe₂ film prepared by using the mechanical exfoliation. Figure 6.10 shows the difference of the peak frequency in the E2 g¹

( )

Γ and A1g

( )

Γ phonon modes. It is approximately 11.1 cm^-1, which is characteristic of monolayer WSe2 [69,144].

在文檔中 摻雜鑭系元素(鏑,釓)氧化鋅與單層二(硫,硒)化鎢薄膜的光譜性質研究 (頁 184-189)