Willem J. P. van Enckevort

Twinning superlattices in indium phosphide nanowires

Semiconducting nanowires offer the possibility of nearly unlimited complex bottom-up design, which allows for new device concepts. However, essential parameters that determine the electronic quality of the wires, and which have not been controlled yet for the III–V compound semiconductors, are the wire crystal structure and the stacking fault density. In addition, a significant feature would be to have a constant spacing between rotational twins in the wires such that a twinning superlattice is formed, as this is predicted to induce a direct bandgap in normally indirect bandgap semiconductors, such as silicon and gallium phosphide. Optically active versions of these technologically relevant semiconductors could have a significant impact on the electronics and optics industry. Here we show first that we can control the crystal structure of indium phosphide (InP) nanowires by using impurity dopants. We have found that zinc decreases the activation barrier for two-dimensional nucleation growth of zinc-blende InP and therefore promotes crystallization of the InP nanowires in the zinc-blende, instead of the commonly found wurtzite, crystal structure. More importantly, we then demonstrate that we can, once we have enforced the zinc-blende crystal structure, induce twinning superlattices with long-range order in InP nanowires. We can tune the spacing of the superlattices by changing the wire diameter and the zinc concentration, and we present a model based on the distortion of the catalyst droplet in response to the evolution of the cross-sectional shape of the nanowires to quantitatively explain the formation of the periodic twinning.

Emergence of a Single Solid Chiral State from a Nearly Racemic Amino Acid Derivative

The evolution of a single chiral solid state is reported for an amino acid derivative starting from a nearly racemic mixture of solid left- and right-handed crystals. Attrition-enhanced dissolution and recrystallization processes based on solubility considerations of the Gibbs−Thomson rule, coupled with solution-phase racemization, drive this near-equilibrium system inexorably to single chirality in the solid phase.

Complete Chiral Symmetry Breaking of an Amino Acid Derivative Directed by Circularly Polarized Light

Circularly polarized light (CPL) emitted from star-forming regions is an attractive candidate as a cause of single chirality in nature. It has remained difficult, however, to translate the relatively small chemical effects observed on irradiation of molecular systems with CPL into high enantiomeric excesses. Here we demonstrate that irradiation of a racemic amino acid derivative with CPL leads to a small amount of chiral induction that can be amplified readily to give an enantiopure solid phase. A racemate composed of equal amounts of left- and right-handed crystals in contact with the irradiated solution is converted completely into crystals of single-handedness through abrasive grinding when racemization is effected in the solution. The rotation sense of the CPL fully determines the handedness of the final solid state. These findings illustrate the potential effectiveness of CPL in the control of molecular asymmetry, which is relevant for the origin of the single chirality inherent to many biological molecules.

The role of surface energies and chemical potential during nanowire growth

We present an approach to quantitatively determine the magnitudes and the variation of the chemical potential in the droplet (Δμ), the solid-liquid (γ(SL)) and the liquid-vapor (γ(LV)) interface energies upon variation of the group III partial pressure during vapor-liquid-solid-growth of nanowires. For this study, we use GaP twinning superlattice nanowires. We show that γ(LV) is the quantity that is most sensitive to the Ga partial pressure (p(Ga)), its dependence on p(Ga) being three to four times as strong as that of γ(SL) or Δμ, and that as a consequence the surface energies are as important in determining the twin density as the chemical potential. This unexpected result implies that surfactants could be used during nanowire growth to engineer the nanowire defect structure and crystal structure.

Crystal Structure Transfer in Core/Shell Nanowires

Structure engineering is an emerging tool to control opto-electronic properties of semiconductors. Recently, control of crystal structure and the formation of a twinning superlattice have been shown for III-V nanowires. This level of control has not been obtained for Si nanowires, the most relevant material for the semiconductor industry. Here, we present an approach, in which a designed twinning superlattice with the zinc blende crystal structure or the wurtzite crystal structure is transferred from a gallium phosphide core wire to an epitaxially grown silicon shell. These materials have a difference in lattice constants of only 0.4%, which allows for structure transfer without introducing extra defects. The twinning superlattices, periodicity, and shell thickness can be tuned with great precision. Arrays of free-standing Si nanotubes are obtained by a selective wet-chemical etch of the core wire.

Complete Chiral Resolution Using Additive-Induced Crystal Size Bifurcation During Grinding

Grinding them down: By using a tailor-made additive, even in the absence of racemization in solution, abrasive grinding can yield an enantiopure solid state. This novel chiral resolution technique is based on an asymmetric bifurcation in the crystal size distribution as a result of stereoselective hampered crystal growth. R = o-tolyl.

Paired twins and [112] morphology in GaP nanowires.

Formation of random as well as periodic planar defects can occur during vapor-liquid-solid growth of nanowires with the zinc-blende crystal structure. Here we investigate the formation of pairs of twin planes in GaP nanowires. In such pairs, the first twin plane is formed at a random position, rapidly followed by the formation of a second twin plane of which the position is directly related to that of the first one. We show that the triangular [112] morphology of the nanowire is a key element in the formation of these twin pairs. We have extended our previous kinetic nucleation model and show that this describes the development of the nanowire morphology and its relation with the formation of single and paired twin planes.

Emergence of single-molecular chirality from achiral reactants

The synthesis of enantiopure molecules from achiral precursors without the need for pre-existing chirality is a major challenge associated with the origin of life. We here show that an enantiopure product can be obtained from achiral starting materials in a single organic reaction. An essential characteristic of this reaction is that the chiral product precipitates from the solution, introducing a crystal–solution interface which functions as an asymmetric autocatalytic system that provides sufficient chiral amplification to reach an enantiopure end state. This approach not only provides more insight into the origin of life but also offers a pathway to acquire enantiopure compounds for industrial applications.

Stability of the polar {111} NaCl crystal face

We present a surface x-ray diffraction determination of the {111} NaCl-liquid interface structure. Using ultrathin water or formamide liquid layers we ascertained that the crystal surface is smooth at an atomic level and is not reconstructed. Our results reveal surprisingly small differences in surface structure between the two cases, which nevertheless lead to dramatic differences in crystal morphology. We determined that the rocksalt {111} surface is Na(+) terminated for both environmental conditions. A quarter to half a monolayer of laterally disordered Cl(-) ions is located on top of a fully ordered Na(+) crystal surface with occupancy 0.75-1.0. This means that the polar surface is stabilized through the formation of an electrochemical double layer.

Magnetite 3D colloidal crystals formed in the early solar system 4.6 billion years ago.

Three-dimensional colloidal crystals made of ferromagnetic particles, such as magnetite (Fe(3)O(4)), cannot be synthesized in principle because of the strong attractive magnetic interaction. However, we discovered colloidal crystals composed of polyhedral magnetite nanocrystallites of uniform size in the range of a few hundred nanometers in the Tagish Lake meteorite. Those colloidal crystals were formed 4.6 billion years ago and thus are much older than natural colloidal crystals on earth, such as opals, which formed about 100 million years ago. We found that the size of each individual magnetite particle determines its morphology, which in turn plays an important role in deciding the packing structure of the colloidal crystals. We also hypothesize that each particle has a flux-closed magnetic domain structure, which reduces the interparticle magnetic force significantly.