Andrew Crowther, Assistant Professor of Chemistry, has published an article entitled "Transition from Molecular Vibrations to Phonons in Atomically Precise Cadmium Selenide Quantum Dots" in the Journal of the American Chemistry Society. Barnard student Rachel Dziatko '18 is a co-author on the article.
The article explores semiconducting nanocrystals, also known as quantum dots, which are composed of hundreds to thousands of atoms in an ordered lattice. Changing nanocrystal size causes a corresponding shift in the color of light absorbed and emitted. These tunable optical properties make nanocrystals promising for displays, solid-state lighting, and next generation energy technologies. Optically excited nanocrystals dissipate energy into their vibrations, so a detailed understanding of nanocrystal vibrational structure is necessary to fully realize their potential. Unfortunately, traditional synthetic approaches generate a distribution of nanocrystal sizes and shapes, precluding a precise characterization of nanocrystal vibrational structure.
Prof. Crowther and Rachel Dziatko (BC ’18), in collaboration with researchers at Columbia University, solve this problem by focusing on a series of three pyramid shaped cadmium selenide nanocrystals that are atomically precise in size and shape and have edges that range from 1.7 to 2.6 nm in length. Investigating this previously unexplored size regime allows them to identify the transition from molecular to bulk vibrations in cadmium selenide nanocrystals for the first time. To do so, they use a micro-Raman spectrometer that measures sample vibrations by analyzing scattered laser light. These measurements, combined with quantum chemical calculations, show that Cd-Se vibrations are distributed throughout the nanocrystal, although the corresponding atomic motion is more localized on either interior or surface atoms. Raman measurements also show that the smallest nanocrystal has a molecular vibrational structure, while the largest nanocrystal has the vibrations of a bulk crystal. This result allows them to identify the edge length of the intermediate-sized nanocrystal, 2.1 nm, as the boundary between a molecular and bulk vibrational structure.
The full article can be found at the following link:
Published abstract below:
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 (Td) 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.