Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization.

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

Electron microscopy imaging of proteins on gallium phosphide semiconductor nanowires.

We have imaged GaP nanowires (NWs) incubated with human laminin, serum albumin (HSA), and blood plasma using both cryo-transmission electron microscopy and synchrotron based X-ray photoemission electron microscopy. This extensive imaging methodology simultaneously reveals structural, chemical and morphological details of individual nanowires and the adsorbed proteins. We found that the proteins bind to NWs, forming coronas with thicknesses close to the proteins' hydrodynamic diameters. We could directly image how laminin is extending from the NWs, maximizing the number of proteins bound to the NWs. NWs incubated with both laminin and HSA show protein coronas with a similar appearance to NWs incubated with laminin alone, indicating that the presence of HSA does not affect the laminin conformation on the NWs. In blood plasma, an intermediate sized corona around the NWs indicates a corona with a mixture of plasma proteins. The ability to directly visualize proteins on nanostructures in situ holds great promise for assessing the conformation and thickness of the protein corona, which is key to understanding and predicting the properties of engineered nanomaterials in a biological environment.

Size and shape dependent few-cycle near-field dynamics of bowtie nanoantennas.

Metal nanostructures can transfer electromagnetic energy from femtosecond laser pulses to the near-field down to spatial scales well below the optical diffraction limit. By combining few-femtosecond laser pulses with photoemission electron microscopy, we study the dynamics of the induced few-cycle near-field in individual bowtie nanoantennas. We investigate how the dynamics depend on antenna size and exact bowtie shape resulting from fabrication. Different dynamics are, as expected, measured for antennas of different sizes. However, we also detect comparable dynamics differences between individual antennas of similar size. With Finite-difference time-domain simulations we show that these dynamics differences between similarly sized antennas can be due to small lateral shape variations generally induced during the fabrication.

Nanoscale Imaging of Local Few-Femtosecond Near-Field Dynamics within a Single Plasmonic Nanoantenna.

The local enhancement of few-cycle laser pulses by plasmonic nanostructures opens up for spatiotemporal control of optical interactions on a nanometer and few-femtosecond scale. However, spatially resolved characterization of few-cycle plasmon dynamics poses a major challenge due to the extreme length and time scales involved. In this Letter, we experimentally demonstrate local variations in the dynamics during the few strongest cycles of plasmon-enhanced fields within individual rice-shaped silver nanoparticles. This was done using 5.5 fs laser pulses in an interferometric time-resolved photoemission electron microscopy setup. The experiments are supported by finite-difference time-domain simulations of similar silver structures. The observed differences in the field dynamics across a single particle do not reflect differences in plasmon resonance frequency or dephasing time. They instead arise from a combination of retardation effects and the coherent superposition between multiple plasmon modes of the particle, inherent to a few-cycle pulse excitation. The ability to detect and predict local variations in the few-femtosecond time evolution of multimode coherent plasmon excitations in rationally synthesized nanoparticles can be used in the tailoring of nanostructures for ultrafast and nonlinear plasmonics.

Manipulating the dynamics of self-propelled gallium droplets by gold nanoparticles and nanoscale surface morphology.

Using in situ surface-sensitive electron microscopy performed in real time, we show that the dynamics of micron-sized Ga droplets on GaP(111) can be manipulated locally using Au nanoparticles. Detailed measurements of structure and dynamics of the surface from microns to atomic scale are done using both surface electron and scanning probe microscopies. Imaging is done simultaneously on areas with and without Au particles and on samples spanning an order of magnitude in particle coverages. Based on this, we establish the equations of motion that can generally describe the Ga droplet dynamics, taking into account three general features: the affinity of Ga droplets to cover steps and rough structures on the surface, the evaporation-driven transition of the surface nanoscale morphology from rough to flat, and the enhanced evaporation due to Ga droplets and Au nanoparticles. Separately, these features can induce either self-propelled random motion or directional motion, but in combination, the self-propelled motion acts to increase the directional motion even if the directional force is 100 times weaker than the random force. We then find that the Au particles initiate a faster native oxide desorption and speed up the rough to flat surface transition in their vicinity. This changes the balance of forces on the Ga droplets near the Au particles, effectively deflecting the droplets from these areas. The model is experimentally verified for the present materials system, but due to its very general assumptions, it could also be relevant for the many other materials systems that display self-propelled random motion.

Alloying and oxidation of in situ produced core-shell Al@Yb nanoalloy particles--an "on-the-fly" study.

Core-shell-structured nanoalloy particles with an Al-dominated interior covered by few Yb monolayers have been fabricated using a vapor-aggregation method involving magnetron sputtering. The radially segregated structure of the Yb-Al nanoparticles has been disclosed by "on-the-fly" photoelectron spectroscopy monitoring of the nanoparticle beam in Yb 4f and Al 2p electron binding energy regions. Both, the binding energy values and the electron microscopy images taken on the deposited nanoparticles, allow estimating their dimensions to be in the 5-10 nm range. The photoelectron spectroscopy results suggest that in these nanoparticles no trivalent Yb--the typical case for the macroscopic Yb-Al alloy--is present. The oxidation of preformed Yb-Al nanoparticles was successfully attempted, leading to the appearance of divalent Yb surface oxide--in contrast to the bulk macroscopic Yb which is trivalent in the oxide. Our results suggest that at intermediate oxygen exposures "sandwich-like" nanoparticles of YbO/Yb/Al were synthesized. At higher O2 exposures, the oxygen seems to penetrate all the way to the Yb-Al interface. The results of the present study have to be considered when photonic applications of Yb-doped garnet nanoparticles are planned.