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

Abstract

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 $1.46<\hslash \omega <1.91$ eV (wavelength $850>\lambda >650$ nm) above the two-photon band gap $E_g^{(QD)}/2$, and at a representative photon energy $\hslash \omega =0.99$ 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.

35 pages, 21 figures, 79 references. The following manuscript was published in ACS Nano. This preprint is the submitted version. The peer-reviewed article can be found here: https://doi.org/10.1021/acsnano.1c11428

References in corpus (2)

• Ultrafast optical control of entanglement between two quantum dot spins
• Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors