The photoluminescence properties of silicon quantum dots may be tailored by surface states via efficient coupling to resonant bulk states. Therefore various wet-chemistry procedures were developed to fabricate silicon quantum dots with adjustable sizes and surface properties. While the energy gap of the Si core is tuned by the size, resonant electronic surface states may be attained by varying the structure and chemical composition of the grafting. The strength of electronic couplings between surface and bulk states determines the timescale of both photo-induced electron transfer from excited surface states to conduction band states and capture of conduction band electrons by surface states. Whereas the origin of photoluminescence and therewith the quantum dot size distribution may be elucidated by stationary luminescence spectroscopy, ultrafast optical spectroscopy techniques, for instance the femtosecond transient absorption spectroscopy, allow to monitor photo-induced electron transfer between surface and bulk states and carrier trapping on the subpicosecond timescale.Temporal evolution of transient absorption spectra of 3-ethenylthiophene terminated Si qdot colloids excited at 390 nm with 150 fs pulses. Surface states as tool to control photoluminescence of Si qdotsOne major objective of Si-based nanophotonics and optoelectronics [1,2] is the development of silicon quantum dots (Si qdots) with optically or electrically controllable luminescence properties. 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 [5-10]. On the other hand, quantum confinement is associated with large carrier wavefunction amplitudes at the quantum dot surfaces that are natively passivated by oxidation. The native passivation shell as an amorphous SiO x layer offers a broad variety of defect structures which are in main optically inactive. Defect structures may efficiently capture the carriers and thereupon reduce the luminescence quantum yield by providing nonradiative recombination centers for the electron-hole pairs [11][12][13]. Therefore Si qdot surfaces are needed to be modified, at least by saturating all dangling bonds and removing defect structures.

In theory, silicon quantum dots (SiQDs) emit enhanced photoluminescence with a size-tunable spectrum in the visible range. In practice, surface states originating from oxide defect structures or organic ligands are strongly involved in exciton relaxation dynamics because the amplitudes of hole and electron wave functions are nonzero at the SiQD surface. In this study, SiQDs with well-defined surface properties were obtained through a wet-chemistry procedure providing SiQDs with adjustable sizes and oxide-free, 3-vinylthiophene-terminated surfaces. The 3-vinylthiophene-terminated SiQDs have a crystalline spherical 2 nm core and were observed to exhibit blue photoluminescence (∼460 nm) with a quantum yield and lifetime of ca. 23% and 1.3 ns, respectively. The interplay between electronically excited molecular states and conduction band states was examined upon direct monitoring of photoexcited carrier dynamics with femtosecond transient absorption spectroscopy. The 3-vinylthiophene ligands were found to act as surface-bound antennae that mediate ultrafast electron transfer or excitation energy transfer across the SiQD interface.