Andrea Pescaglini

Metal nanoparticle–semiconductor nanowire hybrid nanostructures for plasmon-enhanced optoelectronics and sensing

Plasmonic metal nanoparticles have recently attracted increasing interest due to their nanosized dimensions, tunable optical properties in the visible and near-infrared regions of the spectrum and easy manufacturing. Although the optical properties of these sub-wavelength objects arising from plasmonic resonances have been extensively investigated in both isolated and assembled structures, their rational integration in 1D semiconductor-based devices for generation of engineered properties is a novel and vastly unexplored field. In particular, development of metal nanoparticle–1D semiconductor hybrid nanostructures has been hampered by a number of challenges including limited control of component assembly processes and modest theoretical and experimental understanding of fundamental physical phenomena occurring in such hybrids. In this feature article, we describe recent progress in fabrication methods and review the relevant plasmonic properties of metal nanoparticles that can be exploited to manipulate, enhance and optimize the performance of semiconductor nanowire-based devices. Finally, we explore the enhanced properties of hybrid metal nanoparticle–semiconductor nanowire structures and describe their application in optoelectronics and sensing.

Facile Formation of Ordered Vertical Arrays by Droplet Evaporation of Au Nanorod Organic Solutions

Droplet evaporation is a simple method to induce organization of Au nanorods into ordered superstructures. In general, the self-assembly process occurs by evaporation of aqueous suspensions under strictly controlled experimental conditions. Here we present formation of large area ordered vertical arrays by droplet evaporation of Au nanorod organic suspensions. The uncontrolled (free air) evaporation of such suspensions yielded to formation of ordered nanorod domains covering the entire area of a 5 mm diameter droplet. Detailed investigation of the process revealed that nanorods organized into highly ordered vertical domains at the interface between solvent and air on a fast time scale (minutes). The self-assembly process mainly depended on the initial concentration of nanorod solution and required minimal control of other experimental parameters. Nanorod arrays displayed distinct optical properties which were analyzed by optical imaging and spectroscopy and compared to results obtained from theoretical calculations. The potential use of synthesized arrays as surface-enhanced Raman scattering probes was demonstrated with the model molecule 4-aminobenzenthiol.

Selective carrier injection into patterned arrays of pyramidal quantum dots for entangled photon light-emitting diodes

Scalability and foundry compatibility (as apply to conventional silicon-based integrated computer processors, for example) in developing quantum technologies are major challenges facing current research. Here we introduce a quantum photonic technology that has the potential to enable the large-scale fabrication of semiconductor-based, site-controlled, scalable arrays of electrically driven sources of polarization-entangled photons that may be able to encode quantum information. The design of the sources is based on quantum dots grown in micrometre-sized pyramidal recesses along the crystallographic direction (111)B, which theoretically ensures high symmetry of the quantum dots—a requirement for bright entangled-photon emission. A selective electric injection scheme in these non-planar structures allows a high density of light-emitting diodes to be obtained, with some producing entangled photon pairs that also violate Bells inequality. Compatibility with semiconductor fabrication technology, good reproducibility and lithographic position control make these devices attractive candidates for integrated photonic circuits for quantum information processing. Polarization-entangled photons are generated from light-emitting diodes based on site-controlled pyramidal quantum dots. Selective current injection into the vicinity of a quantum dot becomes possible owing to a self-assembled vertical quantum wire.

Synthesis, optical properties and self-assembly of gold nanorods

Noble metal nanostructures of different aspect ratios were synthesised and optically characterised at individual nanorod level. Rayleigh scattering spectroscopy/scanning electron microscopy measurements were performed to uniquely correlate optical signatures with nanorod size and shape. Scattering spectra of nanorods were dominated by the intense longitudinal surface plasmon resonance (SPR) band in the near-infrared part of the spectrum. This band was found to be highly shape and size dependent. Droplet evaporation techniques and application of dielectrophoretic forces have been used to organise nanorod dispersions into ordered arrays. Depending on the technique and nanoparticle size used, nanorods were found to form one, two or three dimensional (1D, 2D and 3D) superstructures. Within these superstructures nanorods organised themselves into end-to-end lines (1D), side-to-side fashion (2D) or hexagonal arrangements (3D).

Controlled assembly of Au nanorods into 1D architectures by electric field assisted deposition

The assembly of Au nanorods of average size 14 × 42 nm was investigated by electric field assisted deposition. The nanorods displayed a rich assembly behavior with formation of one dimensional (1D) architectures under specific experimental conditions. The assembly process was found to be dependent on both the intensity of the applied electric field and the frequency, in contrast to what was expected for metallic particles. A theoretical model based on interpretation of nanorods as core–shell entities was proposed in order to explain the observed behavior. As a result the overall deposition process was represented by a contour plot, where the force acting on the nanorods was displayed as a function of both the electric field and frequency. In particular, an area of the contour plot was identified where the deposition process was driven by generation of localized “hot spots” of high E-field magnitude, leading to formation of well-aligned 1D nanorod architectures bridging the electrode gaps. Electrical characterization showed that 1D architectures displayed tunneling behavior across the inter-nanorod gap. The controlled organization of nanorods into 1D architectures presents opportunities for electronic, sensing and plasmonic applications.

Au nanorod plasmonic superstructures obtained by a combined droplet evaporation and stamping method

A combined droplet evaporation and stamping method is presented for the fabrication of Au nanorod superstructures. Specifically, domains of nanorods parallel to the substrate in a close-packed side-to-side fashion are obtained by evaporation of Au chlorobenzene solutions, followed by stamping of the dried droplet on transparent substrates. To understand and optimize the assembly mechanism, synthetic parameters affecting the droplet evaporation process are carefully investigated. The optical characterization of individual domains shows markedly anisotropic extinction, confirming the high degree of internal order generated by aligned nanorods. In addition, the unique orientation of domains produces a unique distribution of color intensities, which is used for the initial demonstration of a novel plasmonic encoding/decoding system.

Non-resonant Raman spectroscopy of individual ZnO nanowires via Au nanorod surface plasmons

We present a non-resonant Raman spectroscopy study of individual ZnO nanowires mediated by Au nanorod surface plasmons. In this approach, selective excitation of the plasmonic oscillations with radiation energy below the semiconductor bandgap was used to probe surface optical modes of individual ZnO nanowires without simultaneous excitation of bulk phonons modes or band-edge photoluminescence. The development of a reproducible method for decoration of nanowires with colloidal Au nanorods allowed performing an extensive statistical analysis addressing the variability and reproducibility of the Raman features found in the hybrid nanostructures. An estimated field enhancement factor of 103 was calculated, which greatly exceeded previously reported values, and resulted in the detection of a surface optical mode not observable in bare ZnO nanowires under comparable experimental conditions. The role played by Au nanorods in the observed enhancement was investigated both theoretically and experimentally. Specifically, evidence of the superior capabilities in enhancing Raman signals of nanorod longitudinal surface plasmons compared to nanorod transversal surface plasmons is provided. Finite-difference-time domain (FDTD) simulations were used to support the experimental findings and corroborate the use of plasmonic resonances for spectroscopic investigation of individual semiconductor nanostructures.

Dielectrophoretic Self-Assembly of Au Nanorods for Sensing Applications

Anisotropic 1-D metal nanostructures are attractive building block for future optoelectronic nanoscale devices and systems. However, a critical challenge remains the lack of manipulation methods that enable controlled positioning and orientation of metal nanostructures in a fast, reliable and scalable manner. To address this challenge, we explore dielectrophoretic based assembly of discrete gold nanorods and demonstrate site selective assembly and orientation of these rods. The demonstrated optical sensitivity of such large order nanostructures to the local environment opens the way to development of nanoscale sensing devices.

Tuning InP self-assembled quantum structures to telecom wavelength: A versatile original InP(As) nanostructure “workshop”

The influence of hydride exposure on previously unreported self-assembled InP(As) nanostructures is investigated, showing an unexpected morphological variability with growth parameters, and producing a large family of InP(As) nanostructures by metalorganic vapour phase epitaxy, from dome and ring-like structures to double dot in a ring ensembles. Moreover, preliminary microphotoluminescence data are indicating the capped rings system as an interesting candidate for single quantum emitters at telecom wavelengths, potentially becoming a possible alternative to InAs QDs for quantum technology and telecom applications.

Three-dimensional Self-assembled Columnar Arrays of AlInP Quantum Wires for Polarized Micron-sized Amber Light Emitting Diodes

A three-dimensional ordered and self-organized semiconductor system emitting highly polarized light in the yellow-orange visible range (580–650 nm) is presented, comprising self-assembled in-plane AlInP wires vertically stacked in regularly spaced columns. More than 200 wires per column without detectable defect formation could be stacked. Theoretical simulations and temperature-dependent photoluminescence provided a benchmark to engineer multilayered structures showing internal quantum efficiency at room temperature larger than comparable quantum wells emitting at similar wavelengths. Finally, proof-of-concept light-emitting diodes (LED) showed a high degree of light polarization and lower surface parasitic currents than comparable quantum well LEDs, providing an interesting perspective for high-efficiency polarized yellow-orange light-emitting devices.