E. Placidi

Apparent critical thickness versus temperature for InAs quantum dot growth on GaAs(001)

We studied the temperature dependence of the two-dimensional to three-dimensional growth transition in InAs∕GaAs(001) heteroepitaxy by means of reflection high energy electron diffraction and atomic force microscopy. The observed shift of the transition to higher InAs deposition times, at temperatures above 500°C, is not a change of critical thickness for islanding, which instead, is constant in the 450–560°C range. Consequently, In-Ga intermixing and surface and interface strain have a negligible dependence on temperature in this range.

Step erosion during nucleation of InAs∕GaAs(001) quantum dots

We have investigated, by means of atomic force microscopy, the complete evolution of InAs∕GaAs(001) quantum dots as a function of deposited InAs. Direct evidence is found for step erosion by quantum dots nucleated onto the step edge and an estimate of the eroded volume is provided. By studying the quantum dots volume as a function of InAs coverage, we show that the wetting layer contribution is confined within a narrow range of coverage around the two- and three-dimensional transition.

Kinetic aspects of the morphology of self-assembled InAs quantum dots on GaAs(001)

We analyzed by atomic force microscopy self-assembled quantum dots of InAs on GaAs(001) in a series of samples prepared by molecular beam epitaxy (MBE). Two different growth procedures have been applied, namely, the usual continuous growth and the migration-enhanced growth. At equal depositions of InAs, larger than the critical thickness for the two- to three-dimensional transition, marked differences are found in the evolution of the nanoparticle density and volume, despite of the same set of growth parameters were used. Above 2 ML, a small fraction of ripened islands is also present, which is responsible for the nonlinear increase of the total volume of the dots with InAs coverage caused by an anomalous participation of the underlying layers. The different morphologies obtained substantiate the overwhelming role of kinetics on thermodynamics in the nonequilibrium MBE growth.

How kinetics drives the two- to three-dimensional transition in semiconductor strained heterostructures: The case of InAs/GaAs(001)

The two- to three-dimensional growth mode transition in the InAs∕GaAs(001) heterostructure has been investigated by means of atomic force microscopy. The kinetics of the density of three-dimensional islands indicates two transition onsets at 1.45 and 1.59 ML of InAs coverage, corresponding to two separate families, small and large dots. According to the scaling analysis and volume measurements, the transition between the two families of quantum dots and the explosive nucleation of the large ones is triggered by the erosion of the step edges.

Reflection high energy electron diffraction observation of surface mass transport at the two- to three-dimensional growth transition of InAs on GaAs(001)

We have followed by reflection high-energy electron diffraction the nucleation of InAs quantum dots on GaAs(001), grown by molecular-beam epitaxy with growth interruptions. Surface mass transport gives rise, at the critical InAs thickness, to a huge nucleation of three-dimensional islands within 0.2 monolayers (ML). Such surface mass diffusion has been evidenced by observing the transition of the reflection high-energy electron diffraction pattern from two- to three-dimensional during the growth interruption after the deposition of 1.59 ML of InAs. It is suggested that the process is driven by the As2 adsorption-desorption process and by the lowering of the In binding energy due to compressive strain. The last condition is met first in the region surrounding dots at step edges where nucleation predominantly occurs.

Morphological instabilities of the InAs/GaAs(001) interface and their effect on the self-assembling of InAs quantum-dot arrays

The morphology of the InAs/GaAs(001) system has been imaged by atomic force microscopy (AFM) at different stages of the epitaxial growth from the initial formation of a pseudomorphic two-dimensional (2D) interace up to the self-aggregation of InAs quantum dots (QDs). The substrate texture and the dependence of the cation diffusion on the elastic strain field fully control the lateral ordering of the nanoparticles in the self assembling process and determine the final morphology of multistacked InAs QD arrays.

Self-Assembly of a Designed Amyloid Peptide Containing the Functional Thienylalanine Unit

The self-assembly of a peptide based on a sequence from the amyloid beta peptide but incorporating the non-natural amino acid beta-2-thienylalanine (2-Thi) has been investigated in aqueous and methanol solutions. The peptide AAKLVFF was used as a design motif, replacing the phenylalanine residues (F) with 2-Thi units to yield (2-Thi)(2-Thi)VLKAA. The 2-Thi residues are expected to confer interesting electronic properties due to charge delocalization and pi-stacking. The peptide is shown to form beta-sheet-rich amyloid fibrils with a twisted morphology, in both water and methanol solutions at sufficiently high concentration. The formation of a self-assembling hydrogel is observed at high concentration. Detailed molecular modeling using molecular dynamics methods was performed using NOE constraints provided by 2D-NMR experiments. The conformational and charge properties of 2-Thi were modeled using quantum mechanical methods, and found to be similar to those previously reported for the beta-3-thienylalanine analogue. The molecular dynamics simulations reveal well-defined folded structures (turn-like) in dilute aqueous solution, driven by self-assembly of the hydrophobic aromatic units, with charged lysine groups exposed to water.

Self-assembly of a Model Peptide incorporating a Hexa-Histidine sequence attached to an Oligo-Alanine sequence, and binding to Gold NTA/Nickel nanoparticles

Amyloid fibrils are formed by a model surfactant-like peptide (Ala)10-(His)6 containing a hexa-histidine tag. This peptide undergoes a remarkable two-step self-assembly process with two distinct critical aggregation concentrations (cacs), probed by fluorescence techniques. A micromolar range cac is ascribed to the formation of prefibrillar structures, whereas a millimolar range cac is associated with the formation of well-defined but more compact fibrils. We examine the labeling of these model tagged amyloid fibrils using Ni-NTA functionalized gold nanoparticles (Nanogold). Successful labeling is demonstrated via electron microscopy imaging. The specificity of tagging does not disrupt the β-sheet structure of the peptide fibrils. Binding of fibrils and Nanogold is found to influence the circular dichroism associated with the gold nanoparticle plasmon absorption band. These results highlight a new approach to the fabrication of functionalized amyloid fibrils and the creation of peptide/nanoparticle hybrid materials.

The Unexpected Role of Arsenic in Driving the Selective Growth of InAs Quantum Dots on GaAs

Here we show a new effect due to the arsenic flux in the molecular beam epitaxy growth of InAs quantum dots on GaAs(001) at temperatures higher than 500 °C and high As/In flux ratio. We show that, by changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the [110] surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2-5%), the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment, we have developed a kinetic model that explicitly takes into account the coupling between cations (In) and anions (As) and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of quantum dots. We expect a comparable behavior for the anions of other III-V and II-VI compound semiconductors.

Single quantum dot emission by nanoscale selective growth of InAs on GaAs: A bottom-up approach

We report on single dot microphotoluminescence (μPL) emission at low temperature and low power from InAs dots grown by molecular beam epitaxy in nanoscale holes of a SiO2 mask deposited on GaAs(001). By comparing atomic force microscopy measurements with μPL data, we show that the dot sizes inside the nanoholes are smaller than those of the dots nucleated on the extended GaAs surface. PL of dots spans a wide energy range depending on their size and on the thickness and composition of the InGaAs capping layer. Time-resolved PL experiments demonstrate a negligible loss of radiative recombination efficiency, proving highly effective in the site-controlled dot nucleation.