Transition from Molecular Vibrations to Phonons in Atomically Precise Cadmium Selenide Quantum Dots

We use micro-Raman spectroscopy to measure the vibrational structure of the atomically precise cadmium selenide quantum dots Cd₃₅Se₂₀X₃₀L₃₀, Cd₅₆Se₃₅X₄₂L₄₂, and Cd₈₄Se₅₆X₅₆L₅₆. These quantum dots have benzoate (X) and n-butylamine (L) ligands and tetrahedral (T_d) shape with edges that range from 1.7 to 2.6 nm in length. Investigating this previously unexplored size regime allows us to identify the transition from molecular vibrations to bulk phonons in cadmium selenide quantum dots for the first time. Room-temperature Raman spectra have broad CdSe peaks at 175 and 200 cm^-1. Density functional theory calculations assign these peaks to molecular surface and interior vibrational modes, respectively, and show that the interior, surface, and ligand atom motion is strongly coupled. The interior peak intensity increases relative to the surface peak as the cluster size increases due to the relative increase in the polarizability of interior modes with quantum dot size. The Raman spectra do not change with temperature for molecular Cd₃₅Se₂₀X₃₀L₃₀, while the interior peak narrows and shifts to higher energy as temperature decreases for Cd₈₄Se₅₆X₅₆L₅₆, a spectral evolution typical of a phonon. This result shows that the single bulk unit cell contained within Cd₈₄Se₅₆X₅₆L₅₆ is sufficient to apply a phonon confinement model, and that Cd₅₆Se₃₅X₄₂L₄₂, with its 2.1 nm edge length, marks the boundary between molecular vibrations and phonons.