Femtosecond transient absorption spectroscopy of silanized silicon quantum dots

Ultrafast exciton relaxation dynamics in silicon quantum dots

“…The luminescence of silicon quantum dots smaller than free-exciton Bohr's radius (4.3 nm) is attributed to the radiative recombination of carriers confined in the core [3,4]. Quantum confinement allows for tuning the luminescence wavelength in the visible range due to suitably adjusting the Si qdot size [5][6][7][8][9][10]. On the other hand, quantum confinement is associated with large carrier wavefunction amplitudes at the quantum dot surfaces that are natively passivated by oxidation.…”

“…Successful surface termination with suited organic compounds requires the quantitative characterization of photo-excited carrier dynamics and therewith the identification of the different pathways for photoinduced carrier transfers between electronic surface and bulk states. A nanoscopic understanding of ultrafast carrier dynamics in Si qdots on the subpicosecond time scale may be obtained through the use of femtosecond laser spectroscopy [8,14,15].…”

“…2). The asymmetric blue emission band around 27180 cm −1 (370 nm) is ascribed to distinctly weighted energy distributions of surface states which are presumably localized either at defects or at Si=O bonds in the Si-SiO x interface [6] of the structurally disordered SiO x shell [8]. On the other hand, the nearly Gaussian-shaped band at 15100 cm −1 (660 nm) is assigned to radiative recombination of quantum-confined excitons and thereupon associated with a Gaussian size distribution for Si qdots at a mean size of 3.3 nm as providing a Gaussian distribution of the energy gap.…”

“…On the other hand, the nearly Gaussian-shaped band at 15100 cm −1 (660 nm) is assigned to radiative recombination of quantum-confined excitons and thereupon associated with a Gaussian size distribution for Si qdots at a mean size of 3.3 nm as providing a Gaussian distribution of the energy gap. Moreover, the size distribution of the Si qdots determines the relative intensities of both exciton PL and surface-states PL [29], since initially photo-excited conduction-band electrons may be trapped by lower-lying excited surface states for sizes smaller 2.5 nm [6], and electron transfer to the conduction band of Si qdots larger than 3 nm may emerge from initially photo-excited, higher-lying surface states [8].…”

“…Recently, ultrafast exciton dynamics in Si qdots in colloids [8], in free-standing porous silicon films [31,32] or in amorphous SiO 2 films [14] were examined using femtosecond (fs) transient absorption spectroscopy. Matsumoto et al [32] observed two distinct decay dynamics with time constants around 5 ps and 15 ps, which they explained with excitation energy transfer from core states to luminescent surface states and their subsequent PL decay, respectively.…”