Compressive and tensile stress in colloidal CdSe semiconductor quantum dots

Compressive and tensile stress in colloidal CdSe quantum dots (QDs) is examined using resonance Raman spectroscopy. We find that the dispersion of the longitudinal optical phonon mode with size does not follow theoretical calculations based on phonon confinement models. To account for these deviations, the presence of compressive or tensile stress in the QDs is proposed. The influence of surface reconstruction on QD behavior is evidenced by differences in hexadecylamine (HDA) and trioctylphospine oxide (TOP/TOPO) passivation. We find that CdSe QDs passivated by HDA exhibit compressive stress, while CdSe QDs passivated by TOP/TOPO exhibit tensile stress. Evidence is provided that the CdSe-HDA QD stress is directly to a passivant driven surface reconstruction effect.

Figure 1

(A) Optical absorption and photoluminescence of $22.5\mathrm{\AA }$ radius CdSe-HDA QDs. Inset: TEM image of CdSe-HDA QDs. Resonance Raman spectra between 200 and $600{\mathrm{cm}}^{-1}$ of (B) HDA coated (a) 20, (b) 27.5, and (c) $30\mathrm{\AA }$ and (C) TOPO coated (a) 35, (b) 25, and (c) $21.5\mathrm{\AA }$ radius CdSe QDs. The asterisk (*) denotes laser plasma lines.

Figure 2

Size dependent LO phonon for HDA (∎) and TOPO (▴) coated CdSe QDs. The solid lines through the experimental points represent least squares fits of the points to a power law. The other lines represent the theoretical LO dispersion based only on surface tension (solid) and on phonon confinement (dashed-dot). The dotted line represents the bulk LO phonon frequency for CdSe.

Figure 3

Compressive stress as a function of particle size for CdSe-HDA QDs. The solid lines through the data points are a least squares fit of the data from this paper (∎) and from Ref. 6 (●). The dotted line represents the calculated surface stress obtained from Eq. (5). The inset plots the size-dependent tensile stress for CdSe-TOPO QDs. Ordinate and abscissa are same in inset as in full spectra.

Figure 4

(A) Comparison of $25\mathrm{\AA }$ radius CdSe quantum dots passivated with (a) TOPO and (b) ZnS and (B) Change in LO phonon frequency versus change in pressure with addition of ZnS cap. Line is a guide for the eye.