Eleonora Russo-Averchi

Self-assembled quantum dots in a nanowire system for quantum photonics

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Three-Dimensional Multiple-Order Twinning of Self-Catalyzed GaAs Nanowires on Si Substrates

In this paper we introduce a new paradigm for nanowire growth that explains the unwanted appearance of parasitic nonvertical nanowires. With a crystal structure polarization analysis of the initial stages of GaAs nanowire growth on Si substrates, we demonstrate that secondary seeds form due to a three-dimensional twinning phenomenon. We derive the geometrical rules that underlie the multiple growth directions observed experimentally. These rules help optimizing nanowire array devices such as solar or water splitting cells or of more complex hierarchical branched nanowire devices.

Suppression of three dimensional twinning for a 100% yield of vertical GaAs nanowires on silicon

Multiple seed formation by three-dimensional twinning at the initial stages of growth explains the manifold of orientations found when self-catalyzed GaAs nanowires grow on silicon. This mechanism can be tuned as a function of the growth conditions by changing the relative size between the GaAs seed and the Ga droplet. We demonstrate how growing under high V/III ratio results in a 100% yield of vertical nanowires on silicon(111). These results open up the avenue towards the efficient integration of III-V nanowire arrays on the silicon platform.

Photonic–Plasmonic Coupling of GaAs Single Nanowires to Optical Nanoantennas

We successfully demonstrate the plasmonic coupling between metal nanoantennas and individual GaAs nanowires (NWs). In particular, by using dark-field scattering and second harmonic excitation spectroscopy in partnership with analytical and full-vector FDTD modeling, we demonstrate controlled electromagnetic coupling between individual NWs and plasmonic nanoantennas with gap sizes varied between 90 and 500 nm. The significant electric field enhancement values (up to 20×) achieved inside the NW-nanoantennas gap regions allowed us to tailor the nonlinear optical response of NWs by engineering the plasmonic near-field coupling regime. These findings represent an initial step toward the development of coupled metal-semiconductor resonant nanostructures for the realization of next generation solar cells, detectors, and nonlinear optical devices with reduced footprints and energy consumption.

III–V nanowire arrays: growth and light interaction

Semiconductor nanowire arrays are reproducible and rational platforms for the realization of high performing designs of light emitting diodes and photovoltaic devices. In this paper we present an overview of the growth challenges of III-V nanowire arrays obtained by molecular beam epitaxy and the design of III-V nanowire arrays on silicon for solar cells. While InAs tends to grow in a relatively straightforward manner on patterned (111)Si substrates, GaAs nanowires remain more challenging; success depends on the cleaning steps, annealing procedure, pattern design and mask thickness. Nanowire arrays might also be used for next generation solar cells. We discuss the photonic effects derived from the vertical configuration of nanowires standing on a substrate and how these are beneficial for photovoltaics. Finally, due to the special interaction of light with standing nanowires we also show that the Raman scattering properties of standing nanowires are modified. This result is important for fundamental studies on the structural and functional properties of nanowires.

Cantilever magnetometry of individual Ni nanotubes

Recent experimental and theoretical work has focused on ferromagnetic nanotubes due to their potential applications as magnetic sensors or as elements in high-density magnetic memory. The possible presence of magnetic vortex states-states which produce no stray fields-makes these structures particularly promising as storage devices. Here we investigate the behavior of the magnetization states in individual Ni nanotubes by sensitive cantilever magnetometry. Magnetometry measurements are carried out in the three major orientations, revealing the presence of different stable magnetic states. The observed behavior is well-described by a model based on the presence of uniform states at high applied magnetic fields and a circumferential onion state at low applied fields.

Quantum Dot Opto-Mechanics in a Fully Self-Assembled Nanowire

We show that optically active quantum dots (QDs) embedded in MBE-grown GaAs/AlGaAs core-shell nanowires (NWs) are coupled to the NW mechanical motion. Oscillations of the NW modulate the QD emission energy in a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction enables the dynamical tuning of two neighboring QDs into resonance, possibly allowing for emitter-emitter coupling. Both the QDs and the coupling mechanism, i.e. material strain, are intrinsic to the NW structure and do not depend on any functionalization or external field. Such systems open up the prospect of using QDs to probe and control the mechanical state of a NW, or conversely of making a quantum nondemolition readout of a QD state through a position measurement.

Three-dimensional nanoscale study of Al segregation and quantum dot formation in GaAs/AlGaAs core-shell nanowires

GaAs/Al-GaAs core-shell nanowires fabricated by molecular beam epitaxy contain quantum confining structures susceptible of producing narrow photoluminescence (PL) and single photons. The nanoscale chemical mapping of these structures is analyzed in 3D by atom probe tomography (APT). The study allows us to confirm that Al atoms tend to segregate within the AlGaAs shells towards the vertices of the hexagons defining the nanowire cross section. We also find strong alloy fluctuations remaining AlGaAs shell, leading occasionally to the formation of quantum dots (QDs). The PL emission energies predicted in the framework of a 3D effective mass model for a QD analyzed by APT and the PL spectra measured on other nanowires from the same growth batch are consistent within the experimental uncertainties.

Reversal Mechanism of an Individual Ni Nanotube Simultaneously Studied by Torque and SQUID Magnetometry

Using an optimally coupled nanometer-scale SQUID, we measure the magnetic flux originating from an individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same time, we detect the nanotubes volume magnetization using torque magnetometry. We observe both the predicted reversible and irreversible reversal processes. A detailed comparison with micromagnetic simulations suggests that vortexlike states are formed in different segments of the individual nanotube. Such stray-field free states are interesting for memory applications and noninvasive sensing.

Vertical "III-V" V-Shaped Nanomembranes Epitaxially Grown on a Patterned Si[001] Substrate and Their Enhanced Light Scattering

We report on a new form of III-V compound semiconductor nanostructures growing epitaxially as vertical V-shaped nanomembranes on Si(001) and study their light-scattering properties. Precise position control of the InAs nanostructures in regular arrays is demonstrated by bottom-up synthesis using molecular beam epitaxy in nanoscale apertures on a SiO(2) mask. The InAs V-shaped nanomembranes are found to originate from the two opposite facets of a rectangular pyramidal island nucleus and extend along two opposite <111> B directions, forming flat {110} walls. Dark-field scattering experiments, in combination with light-scattering theory, show the presence of distinctive shape-dependent optical resonances significantly enhancing the local intensity of incident electromagnetic fields over tunable spectral regions. These new nanostructures could have interesting potential in nanosensors, infrared light emitters, and nonlinear optical elements.