Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has the potential to open up new opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range $1.46 < \hbar \omega < 1.91$ eV (wavelength $850 > \lambda > 650$ nm) above the two-photon band gap $E_g^{(QD)}/2$, and at a representative photon energy $\hbar \omega = 0.99$ eV ($\lambda = 1250$ nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters $d = 1.8 \pm 0.2$ and $d = 2.3 \pm 0.3$ nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals and the onset of 2PA is observed to be blueshifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bio-imaging using 2PE-PL are simulated in various biological tissues and compared to other quantum dots and molecular fluorophores and found to be superior at greater depths.