Abstract：The fundamental momentum conservation requirement $q\sim0$ for the Raman process is relaxed in the nanocrystallites (NCs), and phonons away from the Brillouin-zone center will be involved in the Raman scattering, which is well-known as the phonon confinement effect in NCs. This usually gives a downshift and asymmetric broadening of the Raman peak in various NCs. Recently, the $A'_1$ mode of 1L MoS$_2$ NCs is found to exhibit a blue shift and asymmetric broadening toward the high-frequency side [Chem. Soc. Rev. 44 (2015) 2757 and Phys. Rev. B 91 (2015) 195411]. In this work, we carefully check this issue by studying Raman spectra of 1L MoS$_2$ NCs prepared by the ion implantation technique in a wide range of ion-implanted dosage. The same confinement coefficient is used for both $E'$ and $A'_1$ modes in 1L MoS$_2$ NCs since the phonon uncertainty in an NC is mainly determined by its domain size. The asymmetrical broadening near the $A'_1$ and $E'$ modes is attributed to the appearance of defect-activated phonons at the zone edge and the intrinsic asymmetrical broadening of the two modes, where the anisotropy of phonon dispersion curves along ${\it \Gamma}$–$K$ and ${\it \Gamma}$–$M$ is also considered. The photoluminescence spectra confirm the formation of small domain size of 1L MoS$_2$ nanocrystallites in the ion-implanted 1L MoS$_2$. This study provides not only an approach to quickly probe phonon dispersion trends of 2D materials away from ${\it \Gamma}$ by the Raman scattering of the corresponding NCs, but also a reference to completely understand the confinement effect of different modes in various nanomaterials.
2016, Vol. 33 
