Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator

{Bi}_{2} {Te}_{3}

and

{Bi}_{2} {Se}_{3}

two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency

(< 30 c m^{- 1})

interlayer vibrational modes are revealed in few-quintuple-layer (QL)

B i_{2} T e_{3} / B i_{2} S e_{3}

2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL

{Bi}_{2} {Te}_{3}

(down to 2QL) and

{Bi}_{2} {Se}_{3}

(down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow

(< 12 c m^{- 1})

frequency peak is observed in 2–4QL

{Bi}_{2} {Te}_{3}

, which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.

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