Abstract
We measure the temperature dependence of breathing-mode acoustic vibrations of semiconductor nanocrystals using low-frequency Raman spectroscopy. In CdSe core-only nanocrystals, the lowest-energy l = 0 mode red-shifts with increasing temperature by ∼5% between 77-300 K. Changes to the interatomic bond distances in the inorganic crystal lattice, with corresponding changes to the bulk modulus and density of the material, contribute to the observed energy shift but do not fully explain its magnitude across all nanocrystal sizes. Invariance of the Raman linewidth over the same temperature range suggests that the acoustic breathing mode is inhomogeneously broadened. The acoustic phonons of CdSe/CdS core-shell composite nanocrystals display similar qualitative behavior. However, for large core-shell nanocrystals, we observe a higher-order Raman peak at approximately twice the energy of the l = 0 mode, which we identify as a higher spherical harmonic-the n = 2, l = 0 eigenmode-rather than a two-phonon scattering event.
Highlights
Semiconductor nanocrystals, commonly known as quantum dots (QDs), are highly tunable, quantum-confined materials with solid-state applications ranging from TVs to solar cells.[1,2] The myriad of applications of these materials stem from the small size of the semiconductor, where excited states are confined to occupy a finite volume and allow only certain discrete energies, yielding a well-defined band-edge transition energy
In order to understand how the acoustic phonon energies of QDs vary as a function of temperature, we synthesized a series of colloidal QDs according to Peng and Peng,[23] where aliquots were removed at various reaction times in order to achieve an array of sizes
For longitudinal optical (LO) phonons in nanocrystals, the phonon linewidth is typically decomposed into a temperature-dependent contribution and a temperature independent contribution, resulting from an intrinsic zone-center phonon linewidth derived from its lifetime and a phonon-confinement effect broadening, respectively.[12]
Summary
Semiconductor nanocrystals, commonly known as quantum dots (QDs), are highly tunable, quantum-confined materials with solid-state applications ranging from TVs to solar cells.[1,2] The myriad of applications of these materials stem from the small size of the semiconductor, where excited states are confined to occupy a finite volume and allow only certain discrete energies, yielding a well-defined band-edge transition energy. The optical properties of QDs are not their only size-dependent feature, ; the acoustic phonons confined to the volume of a nanocrystal exhibit highly sizedependent energies, with phonon frequency roughly inversely proportional to QD radius.[3] These acoustic phonons are known to play a role in QD thermalization,[4] exciton decoherence and dephasing,[5,6] and non-radiative relaxation processes,[7] among others. Temperature dependent frequency shifts and peak widths have been extensively studied for optical phonons in bulk crystals[9,10,11] and nanocrystals,[12,13,14,15,16,17,18] and are modeled using bond anharmonicity models. For CdSe/CdS core–shell nanocrystals, low-temperature Raman spectra reveal that higher order Raman features are the higher spherical harmonics of the fundamental breathing mode rather than multiphonon features
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