InGaAs quantum dot chains grown by twofold selective area molecular beam epitaxy.

Increasing quantum confinement in semiconductor quantum dot systems is essential to perform robust simulations of many-body physics. By combining molecular beam epitaxy and lithographic techniques, we developed an approach consisting of a twofold selective area growth to build quantum dot chains. Starting from 15 nm-thick and 65 nm-wide in-plane In0.53Ga0.47As nanowires on InP substrates, linear arrays of In0.53Ga0.47As quantum dots were grown on top, with tunable lengths and separations. Kelvin probe force microscopy performed at room temperature revealed a change of quantum confinement in chains with decreasing quantum dot sizes, which was further emphasized by the spectral shift of quantum levels resolved in the conduction band with low temperature scanning tunneling spectroscopy. This approach, which allows the controlled formation of 25 nm-thick quantum dots with a minimum length and separation of 30 nm and 22 nm respectively, is suitable for the construction of scalable fermionic quantum lattices. .

Strong and weak polarization-dependent interactions in connected and disconnected plasmonic nanostructures.

We explore numerically and experimentally the formation of hybridized modes between a bright mode displayed by a gold nanodisc and either dark or bright modes of a nanorod - both elements being either separated by a nanometer-size gap (disconnected system) or relied on a metal junction (connected system). In terms of modeling, we compare the scattering or absorption spectra and field distributions obtained under oblique-incidence plane wave illumination with quasi-normal mode computation and an analytical model based on a coupled oscillator model. Both connected and disconnected systems have very different plasmon properties in longitudinal polarization. The disconnected system can be consistently understood in terms of the nature of hybridized modes and coupling strength using either QNMs or coupled oscillator model; however the connected configuration presents intriguing peculiarities based on the strong redistribution of charges implied by the presence of the metal connection. In practice, the fabrication of disconnected or connected configurations depends on the mitigation of lithographic proximity effects inherent to top-down lithography methods, which can lead to the formation of small metal junctions, while careful lithographic dosing allows one to fabricate disconnected systems with a gap as low as 20 nm. We obtained a very good agreement between experimentally measured scattering spectra and numerical predictions. The methods and analyses presented in this work can be applied to a wide range of systems, for potential applications in light-matter interactions, biosensing or strain monitoring.

Engineering a Robust Flat Band in III-V Semiconductor Heterostructures.

Electron states in semiconductor materials can be modified by quantum confinement. Adding to semiconductor heterostructures the concept of lateral geometry offers the possibility to further tailor the electronic band structure with the creation of unique flat bands. Using block copolymer lithography, we describe the design, fabrication, and characterization of multiorbital bands in a honeycomb In0.53Ga0.47As/InP heterostructure quantum well with a lattice constant of 21 nm. Thanks to an optimized surface quality, scanning tunnelling spectroscopy reveals the existence of a strong resonance localized between the lattice sites, signature of a p-orbital flat band. Together with theoretical computations, the impact of the nanopatterning imperfections on the band structure is examined. We show that the flat band is protected against the lateral and vertical disorder, making this industry-standard system particularly attractive for the study of exotic phases of matter.

Precise tailoring of evaporated gold nanocones using electron beam lithography and lift-off.

The ability to fabricate nanocones with precise dimensions is essential for several emerging applications. We demonstrate here a method which can be used to fabricate arrays of gold nanocones with high dimensional precision using lithographic and lift-off means. electron beam (ebeam) writing of a spin-coated PMMA-based bilayer resist deposited onto silicon wafers is used to form a shadow mask. This mask gradually closes as the deposition of gold (using ebeam evaporation) proceeds-the result is arrays of gold nanocones on the silicon wafer surface after lift-off of the resist. Observations using scanning electron microscopy enable a statistical study of the dimensions of 360 gold nanocones-the results demonstrate the high precision of the nanocones dimensions. The fabrication process enables the creation of arrays of nanocones with a base diameter varying from 53.6 ± 2.1 nm to 94.1 ± 2.4 nm, a vertical height ranging from 71.3 ± 4.1 nm to 153.4 ± 3.4 nm, and an apex radius of curvature ranging from 8.4 ± 1.2 nm to 11.6 ± 1.5 nm. The results are compared with the predictions of a deposition model which considers the evolving shadow masking during the gold deposition to compute the nanocone profile.

Size and shape control of a variety of metallic nanostructures using tilted, rotating evaporation and lithographic lift-off techniques.

Here, we demonstrate a simple top-down method for nanotechnology whereby electron beam (ebeam) lithography can be combined with tilted, rotated thermal evaporation to control the topography and size of an assortment of metallic objects at the nanometre scale. In order to do this, the evaporation tilt angle is varied between 1 and 24°. The technique allows the 3-dimensional tailoring of a range of metallic object shapes from sharp, flat bottomed spikes to hollow cylinders and rings-all of which have rotational symmetry and whose critical dimensions are much smaller than the lithographic feature size. The lithographic feature size is varied from 400 nm down to 40 nm. The nanostructures are characterized using electron microscopy techniques-the specific shape can be predicted using topographic modelling of the deposition. Although individual nanostructures are studied here, the idea can easily be extended to fabricate arrays for e.g. photonics and metamaterials. Being a generic technique-depending on easily controlled lithographic and evaporation parameters-it can be readily incorporated into any standard planar process and could be adapted to suit other thin-film materials deposited using physical means.

High speed e-beam lithography for gold nanoarray fabrication and use in nanotechnology.

E-beam lithography has been used for reliable and versatile fabrication of sub-15 nm single-crystal gold nanoarrays and led to convincing applications in nanotechnology. However, so far this technique was either too slow for centimeter to wafer-scale writing or fast enough with the so-called dot on the fly (DOTF) technique but not optimized for sub-15 nm dots dimension. This prevents use of this technology for some applications and characterization techniques. Here, we show that the DOTF technique can be used without degradation in dots dimension. In addition, we propose two other techniques. The first one is an advanced conventional technique that goes five times faster than the conventional one. The second one relies on sequences defined before writing which enable versatility in e-beam patterns compared to the DOTF technique with same writing speed. By comparing the four different techniques, we evidence the limiting parameters for the writing speed. Wafer-scale fabrication of such arrays with 50 nm pitch allowed XPS analysis of a ferrocenylalkyl thiol self-assembled monolayer coated gold nanoarray.

Confinement-modulated junctionless nanowire transistors for logic circuits.

We report the controlled formation of nanoscale constrictions in junctionless nanowire field-effect transistors that efficiently modulate the flow of the current in the nanowire. The constrictions act as potential barriers and the height of the barriers can be selectively tuned by gates, making the device concept compatible with the crossbar geometry in order to create logic circuits. The functionality of the architecture and the reliability of the fabrication process are demonstrated by designing decoder devices.

Large array of sub-10-nm single-grain Au nanodots for use in nanotechnology.

A uniform array of single-grain Au nanodots, as small as 5-8 nm, can be formed on silicon using e-beam lithography. The as-fabricated nanodots are amorphous, and thermal annealing converts them to pure Au single crystals covered with a thin SiO(2) layer. These findings are based on physical measurements, such as atomic force microscopy (AFM), atomic-resolution scanning transmission electron microscopy, and chemical techniques using energy dispersive X-ray spectroscopy. A self-assembled organic monolayer is grafted on the nanodots and characterized chemically with nanometric lateral resolution. The extended uniform array of nanodots is used as a new test-bed for molecular electronic devices.

The collagen assisted self-assembly of silicon nanowires.

The paper reports on self-assembly of silicon nanowire junctions assisted by protocollagen, a low cost soluble long fiber protein and precursor of collagen fibrils. First, the collagen was combed on an octadecyl-terminated silicon surface with gold electrodes. Then the combed surface was exposed to an aqueous suspension of silicon nanowires. In order to increase electrostatic interactions between the positively charged collagen and the nanowires, the nanowires were chemically modified with negatively charged sulfonate groups. The interaction of collagen with the sulfonated nanowires, which mimics the native collagen/heparin sulfate interaction, induced self-assembly of the nanowires localized between gold electrodes. The proof of concept for the formation of spontaneous electrode-nanowire-electrode junctions using collagen as a template was supported by current-voltage measurements.