Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantum dots for optoelectronics applications. However, these higher-order radiative processes are usually quenched in colloidal quantum dots by Auger and other nonradiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantum dots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60 and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics.

Hydrated electrons are the most fundamental anion species, consisting only of electrons and surrounding water molecules. Although hydrated electrons have been extensively studied in the bulk aqueous solutions, even their existence is still controversial at the water surface. Here, we report the observation and characterization of hydrated electrons at the air/water interface using new time-resolved interface-selective nonlinear vibrational spectroscopy. With the generation of electrons at the air/water interface by ultraviolet photoirradiation, we observed the appearance of a strong transient band in the OH stretch region by heterodyne-detected vibrational sum-frequency generation. Through the comparison with the time-resolved spectra at the air/indole solution interface, the transient band was assigned to the vibration of water molecules that solvate electrons at the interface. The analysis of the frequency and decay of the observed transient band indicated that the electrons are only partially hydrated at the water surface, and that they escape into the bulk within 100 ps.

The interface selectivity of vibrational sum frequency generation (VSFG) spectroscopy is explained under the dipole approximation as resulting from the breakdown of inversion symmetry at the interface. From this viewpoint, VSFG is not expected to occur at the interface consisting of centrosymmetric molecules, because the inversion symmetry is preserved even at the interface. In reality, however, VSFG at the nonpolar benzene/air interface has been observed with traditional homodyne-detected VSFG. Here we report a heterodyne-detected VSFG study of the benzene/air interface. The result strongly indicates that VSFG at this interface cannot be explained within the framework of the dipole approximation. The selection rule and polarization dependence of the observed VSFG signal show that the quadrupole transition plays an essential role because of the field discontinuity at the interface. This finding implies the applicability of interface-selective VSFG to the nonpolar interfaces comprising centrosymmetric molecules, which opens a new possibility of VSFG spectroscopy.