Single Elementary Charge Fluctuations on Nanoparticles in Aqueous Solution.

100 years ago, in 1923, the Nobel prize in physics was awarded for measurement of the unit charge. In addition to a profound impact on contemporary physics, this discovery has reshaped our understanding of charge-based interactions in chemistry and biology, ranging from oxidation and ionization to protein folding and metabolism. In a liquid, the discrete nature of the electric charge becomes prominent at the nanoscale when a charge carrier is exchanged between a molecule or a nanoparticle and the surrounding medium. However, our ability to observe the dynamics of such interactions at the level of a single elementary charge is limited due to the abundance of ions in water. Here, we report on the observation of single binding-unbinding events with elementary charge resolution at the surface of a nanoparticle suspended in water. Discrete steps in the electrical charge are revealed by analyzing the motion of optically trapped nanoparticles under the influence of an applied sinusoidal electric field. The measurements are sufficiently fast and long to observe individual (dis)charging events that occur on average every 3 s. Our results offer prospective routes for studying the dynamics of diverse chemical and biological phenomena on the nanoscale with elementary charge resolution.

Uni- and Bidirectional Rotation and Speed Control in Chiral Photonic Micromotors Powered by Light.

Liquid crystalline polymers are attractive materials for untethered miniature soft robots. When they contain azo dyes, they acquire light-responsive actuation properties. However, the manipulation of such photoresponsive polymers at the micrometer scale remains largely unexplored. Here, uni- and bidirectional rotation and speed control of polymerized azo-containing chiral liquid crystalline photonic microparticles powered by light is reported. The rotation of these polymer particles is first studied in an optical trap experimentally and theoretically. The micro-sized polymer particles respond to the handedness of a circularly polarized trapping laser due to their chirality and exhibit uni- and bidirectional rotation depending on their alignment within the optical tweezers. The attained optical torque causes the particles to spin with a rotation rate of several hertz. The angular speed can be controlled by small structural changes, induced by ultraviolet (UV) light absorption. After switching off the UV illumination, the particle recovers its rotation speed. The results provide evidence of uni- and bidirectional motion and speed control in light-responsive polymer particles and offer a new way to devise light-controlled rotary microengines at the micrometer scale.

Broadband Optical Phase Modulation by Colloidal CdSe Quantum Wells.

Two-dimensional (2D) semiconductors are primed to realize a variety of photonic devices that rely on the transient properties of photogenerated charges, yet little is known on the change of the refractive index. The associated optical phase changes can be beneficial or undesired depending on the application, but require proper quantification. Measuring optical phase modulation of dilute 2D materials is, however, not trivial with common methods. Here, we demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using a femtosecond interferometry method. Next, we develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from widely available transient absorption experiments using a modified effective medium algorithm. Our results show that the excitonic features of 2D materials result in broadband, ultrafast, and sizable phase modulation, even extending to the near infrared because of intraband transitions.

Stimulated Emission through an Electron-Hole Plasma in Colloidal CdSe Quantum Rings.

Colloidal CdSe quantum rings (QRs) are a recently developed class of nanomaterials with a unique topology. In nanocrystals with more common shapes, such as dots and platelets, the photophysics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, regardless of the charge carrier density. Here, we show that charge carriers in QRs condense into a hot uncorrelated plasma state at high density. Through strong band gap renormalization, this plasma state is able to produce broadband and sizable optical gain. The gain is limited by a second-order, yet radiative, recombination process, and the buildup is counteracted by a charge-cooling bottleneck. Our results show that weakly confined QRs offer a unique system to study uncorrelated electron-hole dynamics in nanoscale materials.

Measurement of the amplitude and phase of the electrophoretic and electroosmotic mobility based on fast single-particle tracking.

The electrophoretic mobility of micron-scale particles is of crucial importance in applications related to pharmacy, electronic ink displays, printing, and food technology as well as in fundamental studies in these fields. Particle mobility measurements are often limited in accuracy because they are based on ensemble averages and because a correction for electroosmosis needs to be made based on a model. Single-particle approaches are better suited for examining polydisperse samples, but existing implementations either require multiple measurements to take the effect of electroosmosis into account or are limited in accuracy by short measurement times. In this work, accurate characterization of monodisperse and polydisperse samples is achieved by measuring the electrophoretic mobility on a particle-to-particle basis while suppressing electroosmosis. Electroosmosis can be suppressed by measuring in the middle of a microchannel while applying an AC voltage with a sufficiently high frequency. An accurate measurement of the electrophoretic mobility is obtained by analyzing the oscillating particle motion for 1.5s per particle with a high-speed camera measuring at 850Hz , synchronized to the applied electric field. Attention is paid to take into account the effect of the rolling shutter and the non-uniform sampling in order to obtain the accurate amplitude and phase of the electrophoretic mobility. The accuracy of method is experimentally verified and compared with a commercial apparatus for polystyrene microspheres in water. The method is further demonstrated on a range of particle materials and particle sizes and for a mixture of positively and negatively charged particles.

Waveguiding of Photoluminescence in a Layer of Semiconductor Nanoparticles.

Semiconductor nanoparticles (SNPs), such as quantum dots (QDs) and core/shell nanoparticles, have proven to be promising candidates for the development of next-generation technologies, including light-emitting diodes (LEDs), liquid crystal displays (LCDs) and solar concentrators. Typically, these applications use a sub-micrometer-thick film of SNPs to realize photoluminescence. However, our current knowledge on how this thin SNP layer affects the optical efficiency remains incomplete. In this work, we demonstrate how the thickness of the photoluminescent layer governs the direction of the emitted light. Our theoretical and experimental results show that the emission is fully outcoupled for sufficiently thin films (monolayer of SNPs), whereas for larger thicknesses (larger than one tenth of the wavelength) an important contribution propagates along the film that acts as a planar waveguide. These findings serve as a guideline for the smart design of diverse QD-based systems, ranging from LEDs, where thinner layers of SNPs maximize the light outcoupling, to luminescent solar concentrators, where a thicker layer of SNPs will boost the efficiency of light concentration.

Dual Light and Temperature Responsive Micrometer-Sized Structural Color Actuators.

Externally induced color- and shape-changes in micrometer-sized objects are of great interest in novel application fields such as optofluidics and microrobotics. In this work, light and temperature responsive micrometer-sized structural color actuators based on cholesteric liquid-crystalline (CLC) polymer particles are presented. The particles are synthesized by suspension polymerization using a reactive CLC monomer mixture having a light responsive azobenzene dye. The particles exhibit anisotropic spot-like and arc-like reflective colored domains ranging from red to blue. Electron microscopy reveals a multidirectional asymmetric arrangement of the cholesteric layers in the particles and numerical simulations elucidate the anisotropic optical properties. Upon light exposure, the particles show reversible asymmetric shape deformations combined with structural color changes. When the temperature is increased above the liquid crystal-isotropic phase transition temperature of the particles, the deformation is followed by a reduction or disappearance of the reflection. Such dual light and temperature responsive structural color actuators are interesting for a variety of micrometer-sized devices.

Tunable top-down fabrication and functional surface coating of single-crystal titanium dioxide nanostructures and nanoparticles.

Titanium dioxide (TiO2) is a key component of diverse optical and electronic applications that exploit its exceptional material properties. In particular, the use of TiO2 in its single-crystalline phase can offer substantial advantages over its amorphous and polycrystalline phases for existing and yet-to-be-developed applications. However, the implementation of single-crystal TiO2 has been hampered by challenges in its fabrication and subsequent surface functionalization. Here, we introduce a novel top-down approach that allows for batch fabrication of uniform high-aspect-ratio single-crystal TiO2 nanostructures with targeted sidewall profiles. We complement our fabrication approach with a functionalization strategy that achieves dense, uniform, and area-selective coating with a variety of biomolecules. This allows us to fabricate single-crystal rutile TiO2 nanocylinders tethered with individual DNA molecules for use as force- and torque-transducers in an optical torque wrench. These developments provide the means for increased exploitation of the superior material properties of single-crystal TiO2 at the nanoscale.