Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

Carrier multiplication by generation of two or more electron-hole pairs following the absorption of a single photon may lead to improved photovoltaic efficiencies and has been observed in nanocrystals made from a variety of semiconductors, including silicon. However, with few exceptions, these reports have been based on indirect ultrafast techniques. Here, we present evidence of carrier multiplication in closely spaced silicon nanocrystals contained in a silicon dioxide matrix by measuring enhanced photoluminescence quantum yield. As the photon energy increases, the quantum yield is expected to remain constant, or to decrease as a result of new trapping and recombination channels being activated. Instead, we observe a step-like increase in quantum yield for larger photon energies that is characteristic of carrier multiplication. Modelling suggests that carrier multiplication is occurring with high efficiency and close to the energy conservation limit.

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Dendritic spines are the primary site of excitatory synaptic input onto neurons, and are biochemically isolated from the parent dendritic shaft by their thin neck. However, due to the lack of direct electrical recordings from spines, the influence that the neck resistance has on synaptic transmission, and the extent to which spines compartmentalize voltage, specifically excitatory postsynaptic potentials, albeit critical, remains controversial. Here, we use quantum-dot-coated nanopipette electrodes (tip diameters ~15–30 nm) to establish the first intracellular recordings from targeted spine heads under two-photon visualization. Using simultaneous somato-spine electrical recordings, we find that back propagating action potentials fully invade spines, that excitatory postsynaptic potentials are large in the spine head (mean 26 mV) but are strongly attenuated at the soma (0.5–1 mV) and that the estimated neck resistance (mean 420 MΩ) is large enough to generate significant voltage compartmentalization. Nanopipettes can thus be used to electrically probe biological nanostructures.

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The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the $\Gamma$-point decreases with size reduction: quantum confinement enhances radiative recombination across the direct bandgap and introduces its "red" shift for smaller grains. We postulate to identify the frequently reported efficient blue emission (F-band) from Si nanocrystals with this zero-phonon recombination. In a dedicated experiment, we confirm the "red" shift of the F-band, supporting the proposed identification

We report results of time-resolved induced absorption (IA) spectroscopy on Si nanocrystals (Si NCs) embedded in a SiO 2 matrix. In line with theoretical modeling, the IA amplitude decreases with probing photon energy, however only until a certain threshold value. For larger photon energies, an increase of IA is observed. This unexpected behavior is interpreted in terms of the self-trapped exciton state whose formation in Si NCs was put forward some time ago based on theoretical considerations. Here, we present a direct experimental confirmation of this supposition. Silicon nanocrystals (NCs) 1 are frequently investigated for their interesting optical properties and a wide variety of potential applications in optoelectronics, 2-4 photovoltaics, [5][6][7] and the medical field. 8 In particular, the photoluminescence (PL) of Si NCs has been thoroughly characterized by experiment 9,10 and extensively modeled by theory using the ab initio approach 11 as well as the semiempirical methods: pseudopotential, 12 tight-binding, 13,14 and effective mass approximation. 15 In that way, opening of the (indirect) band gap and enhancement of the radiative rate of band-to-band recombination have been firmly established. For oxygenterminated Si NCs a peculiarity has been found: for smaller diameters d NC < 2.5 nm the blueshift in PL spectrum could not be observed, with PL energy stabilizing in the visible range. This has been explained in terms of formation of oxygenrelated defects at the surface of NCs, with levels appearing in the band gap and participating in the recombination of carriers. 16 Specific microscopic details of these defects are not known, but oxygen is well known to form electrically active defects in bulk Si. 17,18 Among other possibilities, formation of a self-trapped exciton state (STE) has been proposed. 19 Support for the existence of the STE state facilitating photon emission in small oxygen-terminated Si NCs was inferred only indirectly from steady-state PL experiments-predominantly from the aforementioned stabilization of the quantum confinement induced blueshift of PL and from the temperature dependence of PL intensity and lifetime. 20 It is fair to say that an experimental evidence directly confirming formation of the STE is still missing.In order to investigate formation and characteristics of the STE state, carrier dynamics directly after photoexcitation need to be investigated. This is best accomplished by means of ultrafast induced absorption (IA) and PL up-conversion spectroscopy. Past investigations revealed that carrier dynamics in Si NCs exhibits always a fast multiexponential decay. [21][22][23][24] This illustrates a variety of carrier relaxation pathways, with individual components assigned to trapping, 25 carrier-carrier scattering, Auger energy transfer between electrons and holes, 14 phonon-assisted cooling, and no-phonon radiative recombination. 10 In that landscape, formation of the STE state has been related to trapping to defects at the surface of Si NCs, competing with carrier cooling....

We report on investigations of optical carrier generation in silicon nanocrystals embedded in an SiO 2 matrix. Carrier relaxation and recombination processes are monitored by means of time-resolved induced absorption, using a conventional femtosecond pump-probe setup for samples containing different average sizes of nanocrystals (d NC = 2.5-5.5 nm). The electron-hole pairs generated by the pump pulse are probed by a second pulse over a broad spectral range (E probe = 0.95-1.35 or 1.6-3.25 eV), by which information on excited states is obtained. Under the same excitation conditions, we observe that the induced absorption intensity in the near-infrared range is a factor of ∼10 higher than in the visible range. To account for these observations, we model the spectral dependence of the induced absorption signal using an empirical sp 3 d 5 s * tight-binding technique, by which the spectrum can be well reproduced up to a certain threshold. For probe photon energies above this threshold (dependent on nanocrystal size), the induced absorption signal is found to feature a long-standing component, whereas the induced absorption signal for probe photon energies below this value vanishes within 0.5 ns. We explain this by self-trapping of excitons on surface-related states.