Tuning Single Quantum Dot Emission with a Micromirror

The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate

Distance and Wavelength Dependent Quenching of Molecular Fluorescence by Au@SiO₂ Core–Shell Nanoparticles

Gold nanoparticles and nearby fluorophores interact <i>via</i> electromagnetic coupling upon light excitation. We determine the distance and wavelength dependence of this coupling theoretically and experimentally <i>via</i> steady-state and time-resolved fluorescence spectroscopy. For the first time, the fluorescence quenching of four different dye molecules, which absorb light at different wavelengths across the visible spectrum and into the near-infrared, is studied using a rigid silica shell as a spacer. A comprehensive experimental determination of the distance dependence from complete quenching to no coupling is carried out by a systematic variation of the silica shell thickness. Electrodynamic theory predicts the observed quenching quantitatively in terms of energy transfer from the molecular emitter to the gold nanoparticle. The plasmonic field enhancement in the vicinity of the 13 nm gold nanoparticles is calculated as a function of distance and excitation wavelength and is included in all calculations. Relative radiative and energy transfer rates are determined experimentally and are in good agreement with calculated rates. We demonstrate and quantify the severe effect of dye–dye interactions on the fluorescence properties of dyes attached to the surface of a silica nanoparticle in control experiments. This allows us to determine the experimental conditions, under which dye–dye interactions do not affect the experimental results

Unconventional Janus Properties of Enokitake-like Gold Nanowire Films

We report on unconventional Janus material properties of vertically aligned gold nanowire films that conduct electricity and interact with light and water in drastically different ways on its two opposing sides. These Janus-like properties originate from enokitake-like nanowire structures, causing the nanoparticle side (“head”) to behave like bulk gold, yet the opposing nanowire side (“tail”) behaves as discontinuous nanophases. Due to this Janus film structure, its head side is hydrophilic but its tail side is hydrophobic; its head side reflects light like bulk gold, yet its tail side is a broadband superabsorber; its tail side is less conductive but with tunable resistance. More importantly, the elastomer-bonded Janus film exhibits unusual mechatronic properties when being stretched, bent, and pressed. The tail-bonded elastomeric sheet can be stretched up to ∼800% strain while remaining conductive, which is about 10-fold that of head-bonded film. In addition, it is also more sensitive to bending forces and point loads than the corresponding tail-bonded film. We further demonstrate the versatility of nanowire-based Janus films for pressure sensors using bilayer structures in three different assembly layouts

Unconventional Janus Properties of Enokitake-like Gold Nanowire Films

We report on unconventional Janus material properties of vertically aligned gold nanowire films that conduct electricity and interact with light and water in drastically different ways on its two opposing sides. These Janus-like properties originate from enokitake-like nanowire structures, causing the nanoparticle side (“head”) to behave like bulk gold, yet the opposing nanowire side (“tail”) behaves as discontinuous nanophases. Due to this Janus film structure, its head side is hydrophilic but its tail side is hydrophobic; its head side reflects light like bulk gold, yet its tail side is a broadband superabsorber; its tail side is less conductive but with tunable resistance. More importantly, the elastomer-bonded Janus film exhibits unusual mechatronic properties when being stretched, bent, and pressed. The tail-bonded elastomeric sheet can be stretched up to ∼800% strain while remaining conductive, which is about 10-fold that of head-bonded film. In addition, it is also more sensitive to bending forces and point loads than the corresponding tail-bonded film. We further demonstrate the versatility of nanowire-based Janus films for pressure sensors using bilayer structures in three different assembly layouts