Defect-Induced Vibration Modes of ${\mathrm{Ar}}^{+}$-Irradiated ${\mathrm{MoS}}_2$

The ${\mathrm{Ar}}^{+}$-irradiated molybdenum-disulfide (${\mathrm{MoS}}_2$) surface is studied by means of Raman spectroscopy and first-principles calculation. This experimental study reveals that ${\mathrm{Ar}}^{+}$ irradiation gives rise to satellite peaks at the lower-frequency side of the Raman-active $E_{2g}^1$ and $A_{1g}$ modes of ${\mathrm{MoS}}_2$ and a new peak at approximately $450{\mathrm{cm}}^{-1}$. We calculate the phonon modes and Raman spectra of defective ${\mathrm{MoS}}_2$ systems from first principles, and show that Mo and S vacancies give rise to such satellite peaks. These satellite peaks are a modulation of the $E_{2g}^1$ and $A_{1g}$ modes, described in terms of localization and scattering of vibration modes. The new peak at $450{\mathrm{cm}}^{-1}$, however, is a unique signature of the S vacancy. At low irradiation doses, the S vacancy is the dominant defect, whereas for large irradiation doses, the satellite peaks overshadow the ${\mathrm{MoS}}_2$ peaks, which we show to be typical for the Mo vacancy and ${\mathrm{MoS}}_6$ vacancy cluster. We thus show that Raman spectroscopy can be used not only to observe defects in two-dimensional materials, but also to identify the type of the defects.

Figure 1

Raman spectra of pristine bulk ${\mathrm{MoS}}_2$ (bottom) and ${\mathrm{Ar}}^{+}$-irradiated bulk ${\mathrm{MoS}}_2$ at fluences of $5.65\times {10}^{14}\mathrm{ions}/{\mathrm{cm}}^2$ (middle) and $2.26\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$ (top).

Figure 2

Raman spectra of chemically exfoliated bilayer ${\mathrm{MoS}}_2$ flakes before (bottom) and after ${\mathrm{Ar}}^{+}$ irradiation at the fluence of $2.3\times {10}^{15}\mathrm{ions}/{\mathrm{cm}}^2$ (top). Notice that the bilayer intensity is multiplied by a factor of 2 ($\times 2$) due to the weakening of the Raman signal.

Figure 3

(a) The slab model of the $5\times 5$ monolayer ${\mathrm{MoS}}_2$. The slab model is extended from the primitive cell in plane. The top views are of (b) pristine, (c) Mo vacancy, (d) S vacancy, and (e) ${\mathrm{MoS}}_6$ vacancy structures. Each vacancy is placed at the center of the slab model.

Figure 4

Calculated Raman spectra of monolayer pristine ${\mathrm{MoS}}_2$. The insets show normal displacements and their vibration frequency of the $E_{2g}^1$ mode and the $A_{1g}$ mode. The envelope of the calculated Raman spectra is evaluated by smearing the peaks with half width of $5{\mathrm{cm}}^{-1}$.

Figure 5

Calculated Raman spectra and lattice vibrations of defective ${\mathrm{MoS}}_2$ with (a) Mo vacancy, (b) S vacancy, and (c) ${\mathrm{MoS}}_6$ vacancy cluster. Envelopes of each Raman spectra are evaluated by smearing the peaks with half width $5{\mathrm{cm}}^{-1}$. Calculated Raman peaks are labeled by numbers and each associated lattice vibration is illustrated in the right column. Note that the displacements in plane along the surface are top viewed, while those out of plane are side viewed.

Figure 6

Spatial distribution of the atomic displacements schematically shown in in-plane and out-of-plane modes. The background colored envelopes show the amplitude of the displacement. The in-plane mode has a weak amplitude near the vacancy while the out-of-plane mode only has amplitude adjacent to the vacancy (cf. the right-hand columns in Fig. 5 for each mode).