Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots

Abstract

Resonance Raman spectra and excitation profiles have been measured and semiquantitatively modeled for core/shell quantum dots consisting of 2.7 nm diameter zincblende CdSe cores and thin (0.5 nm) or thick (1.6 nm) CdS shells. The Raman spectra show previously reported trends of increased peak frequency for both the CdSe and the CdS longitudinal optical (LO) phonons with increasing shell thickness. We also find a strong dependence of the peak CdS frequency on excitation energy and a large discrepancy between the experimental frequency of the CdSe+CdS combination band and the sum of the corresponding fundamental frequencies. This suggests that the dominant transitions at high excitation energies are localized on either the CdSe core or the CdS shell and thereby cannot enhance combination band transitions between core and shell. The CdS to CdSe Raman intensity ratios at high excitation energies further support this picture. The electron-phonon coupling for the CdSe LO phonon in the lowest excitonic transition is slightly weaker in the core/shell structures than in pure CdSe quantum dots, contrary to expectations for the Fröhlich coupling mechanism. Possible explanations for this discrepancy are discussed.

Comparison of three empirical force fields for phonon calculations in CdSe quantum dots

“…CdSe QDs in the size range from 2.8-5.2 nm, when excited near the lowest excitonic absorption, 29,[31][32][33] show a single well defined Raman peak in the LO phonon region with a frequency of 203-207 cm -1 and a full width at half maximum that does not exceed about 20 cm -1 including the effects of instrumental broadening and the distribution of sizes and shapes found in real samples. All three potentials give Raman spectra in reasonably good agreement with experiment for the larger QDs (Fig.…”

“…45 The present treatment also ignores vibrational anharmonicities and calculates the vibrational properties at zero temperature. The measured anharmonicity of the LO phonon mode in CdSe nanocrystals is very small as estimated from overtone frequencies, 32 although the temperature dependence of the LO phonon frequency at higher temperatures suggests that anharmonic coupling to lower-frequency phonons is not negligible. 49,50 Again, while finitetemperature effects may modify the quantitative results obtained here, it is likely that the qualitative behavior of the force fields is adequately captured by the zero-temperature calculations.…”

“…Calculations on nanocrystals were carried out using the general methods described in our earlier publications. [31][32][33]37 Briefly, a bulk wurtzite CdSe lattice was generated, an origin and a radius were chosen, and all atoms beyond that radius were eliminated, as were any remaining surface atoms having fewer than two nearest neighbors. The General Utility Lattice Program (GULP 4.0) 38 was then used to find the equilibrium geometry and the harmonic vibrational frequencies and normal modes with each of the three empirical force fields.…”

“…31,32 Briefly, the electron and hole wavefunctions were calculated using the effective mass approximation particle in a sphere model and the electric field produced by the resulting electron and hole charge distributions was used to calculate the electron-phonon coupling for each vibrational mode via the Fröhlich mechanism. For these comparisons, S-D mixing in the valence band was not included and the electron-hole interaction was not considered in calculating the excitonic wavefunctions.…”

“…These have been employed chiefly in molecular dynamics simulations of processes such as surface reconstruction and phase transitions, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] but there has also been interest in using empirical potentials to calculate the geometries and vibrations of semiconductor nanocrystals ("quantum dots" when nearly spherical). [20][21][22][23][24][25][26][27][28] Our group has been using quantitative resonance Raman intensity analysis to probe electron-phonon coupling in 4 CdSe-containing quantum dots (QDs), [29][30][31][32][33] and full interpretation of these data requires knowledge of the phonon frequencies and modes. While the vibrational properties of molecules and nanomaterials are determined largely by the masses and connectivities of the atoms, the details can be quite dependent upon the specific force field used.…”