Pressure-induced electronic coupling in CdSe semiconductor quantum dots

While previous high-pressure optical and vibrational studies on CdSe quantum dots (QD’s) have observed no evidence for size-dependent scaling laws in the experimental pressure behavior, we have found that low-pressure studies exhibit anomalous pressure dependencies that can be analyzed in two specific size regimes $(r<\mathrm{}{a}_0$ and $r\sim a_0,$ where r is the particle radius and $a_0$ is the Bohr radius). This corresponds to the limits of strong and weak confinement of the exciton for CdSe. By using photoluminescence and resonance Raman spectroscopies, we find optical and vibrational coefficients (82 meV ${\mathrm{GPa}}^{\mathrm{-}1}$ and 3.51 ${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathrm{GPa}}^{\mathrm{-}1}$, respectively, for 22.5 Å radius dots) that significantly deviate from reported bulk values (37 meV ${\mathrm{GPa}}^{\mathrm{-}1}$) when $r<\mathrm{}{a}_0.$ At the largest QD sizes studied $(r=40\mathrm{}\AA ),$ bulk-like pressure dependence is observed as expected. We believe the anomalous size dependent pressure behavior arises from changes in the nature of the electron-phonon (el-ph) coupling due to polaronic coupling in the QD lattice. Analyses of longitudinal optical mode Grüneisen parameters $\left({\gamma }_{\mathrm{LO}}\right)$ show the expected el-ph coupling strengths derived from ${\gamma }_{\mathrm{LO}}$ agree well with calculated values for el-ph coupling strengths in CdSe QD’s. An empirical pressure-dependent model is proposed which argues that polaronic decoupling occurs following a $1/r$ scaling law, which may be due to changes in el-ph coupling in these dimensionally restricted materials.