Anisotropic-Strain-Induced Band Gap Engineering in Nanowire-Based Quantum Dots

Prediction of orientation relationships and interface structures between α-, β-, γ-FeSi2 and Si phases

A pure crystallogeometrical approach is proposed for predicting orientation relationships, habit planes and atomic structures of the interfaces between phases, which is applicable to systems of low-symmetry phases and epitaxial thin film growth. The suggested models are verified with the example of epitaxial growth of α-, γ- and β-FeSi₂ silicide thin films on silicon substrates. The density of near-coincidence sites is shown to have a decisive role in the determination of epitaxial thin film orientation and explains the superior quality of β-FeSi₂ thin grown on Si(111) over Si(001) substrates despite larger lattice misfits. Ideal conjunctions for interfaces between the silicide phases are predicted and this allows for utilization of a thin buffer α-FeSi₂ layer for oriented growth of β-FeSi₂ nanostructures on Si(001). The thermal expansion coefficients are obtained within quasi-harmonic approximation from the DFT calculations to study the influence of temperature on the lattice strains in the derived interfaces. Faster decrease of misfits at the α-FeSi₂(001)||Si(001) interface compared to γ-FeSi₂(001)||Si(001) elucidates the origins of temperature-driven change of the phase growing on silicon substrates. The proposed approach guides from bulk phase unit cells to the construction of the interface atomic structures and appears to be a powerful tool for the prediction of interfaces between arbitrary phases for subsequent theoretical investigation and epitaxial film synthesis.

Room-Temperature Sustainable Synthesis of Selected Platinum Group Metal (PGM = Ir, Rh, and Ru) Nanocatalysts Well-Dispersed on Porous Carbon for Efficient Hydrogen Evolution and Oxidation

Electroless deposition via a spontaneous redox reaction between the metal precursor and support is believed to be a promising approach for the syntheses of supported metal nanoparticles (SMNPs). However, its widespread applications are significantly prohibited by the low reductivity and high cost of support. To overcome these shortcomings, a porous carbon (PC) is herein developed as a promising matrix for the electroless deposition of metal NPs. Benefiting from abundant oxygen-based surface functional groups, the PC shows stronger reducibility (low redox potential) than conventional carbon substrate such as carbon nanotubes or graphene oxide, enabling a facile electroless deposition of Ir, Rh, and Ru NPs on its surface. These SMNPs exhibit an impressive electrocatalytic activity for the hydrogen evolution reaction (HER) or hydrogen oxidation reaction (HOR). For example, the Rh NP/PC can deliver an HER current density of 10 mA cm^-2 with a small overpotential of 21 mV in 0.5 m H₂ SO₄ , while the Ru NP/PC exhibits excellent HOR activity in 0.1 m KOH in terms of high mass and surface specific exchange current density of 263 A g^-1 _Ru and 0.227 mA cm^-2 _Ru . The present strategy may open up opportunities for mass production of efficient supported NPs for diverse applications.

III-V Integration on Si(100): Vertical Nanospades

III-V integration on Si(100) is a challenge: controlled vertical vapor liquid solid nanowire growth on this platform has not been reported so far. Here we demonstrate an atypical GaAs vertical nanostructure on Si(100), coined nanospade, obtained by a nonconventional droplet catalyst pinning. The Ga droplet is positioned at the tip of an ultrathin Si pillar with a radial oxide envelope. The pinning at the Si/oxide interface allows the engineering of the contact angle beyond the Young-Dupré equation and the growth of vertical nanospades. Nanospades exhibit a virtually defect-free bicrystalline nature. Our growth model explains how a pentagonal twinning event at the initial stages of growth provokes the formation of the nanospade. The optical properties of the nanospades are consistent with the high crystal purity, making these structures viable for use in integration of optoelectronics on the Si(100) platform.

Vapor Phase Growth of Semiconductor Nanowires: Key Developments and Open Questions

Nanowires are filamentary crystals with a tailored diameter that can be obtained using a plethora of different synthesis techniques. In this review, we focus on the vapor phase, highlighting the most influential achievements along with a historical perspective. Starting with the discovery of VLS, we feature the variety of structures and materials that can be synthesized in the nanowire form. We then move on to establish distinct features such as the three-dimensional heterostructure/doping design and polytypism. We summarize the status quo of the growth mechanisms, recently confirmed by in situ electron microscopy experiments and defining common ground between the different synthesis techniques. We then propose a selection of remaining defects, starting from what we know and going toward what is still to be learned. We believe this review will serve as a reference for neophytes but also as an insight for experts in an effort to bring open questions under a new light.

Tuning adatom mobility and nanoscale segregation by twin formation and polytypism

Nanoscale variations in the composition of an Al _x Ga_1-x As shell around a GaAs nanowire affect the nanowire functionality and can lead to the formation of localized quantum emitters. These composition fluctuations can be the consequence of variations of crystal phase and/or nanoscale adatom mobility. By applying electron-microscopy-related techniques we correlate the optical, compositional and structural properties at the nanoscale on the same object. The results indicate a clear correlation between the twin density in the nanowire and the quantum-emitter density as well as a significant redshift in the emission. We propose that twinning increases nanoscale segregation effects in ternary alloys. An additional redshift in the emission can be explained by the staggered band alignment between wurtzite and zinc-blende phases. This work opens new avenues in the achievement of homogeneous ternary and quaternary alloys in nanowires and in the engineering of the segregation effects at the nanoscale.