E. Vlieg

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.

From Ostwald Ripening to Single Chirality

A century ago Wilhelm Ostwald received the Nobel Prize for Chemistry. Although Ostwald was never significantly involved with the phenomenon of chirality, one of his discoveries, Ostwald ripening, is thought to be involved in a recently discovered method in which grinding-induced attrition is used to transform racemic conglomerates virtually quantitatively into a single enantiomer. In this Minireview the basic concepts developed by Ostwald will be introduced, followed by a summary of the current status of grinding-induced asymmetric transformations. We will see how close Ostwald himself came to discovering this technique.

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.

Angle Calculations for a Six-Circle Surface X-ray Diffractometer

Angle calculations are presented for a new type of diffractometer suitable for surface X-ray diffraction. The new geometry results from combining the four-circle and the z-axis geometries and involves six circles. Instruments based on this concept can be operated in different diffraction geometries. Compared to the four-circle and z-axis geometries, a larger fraction of the reciprocal space perpendicular to the sample is available to experiments when all six circles are used. This results in an improved out-of-plane resolution in any X-ray structure study. In each geometry, either the angle of incidence or the outgoing angle of the X-rays can be chosen. At the same time, the surface normal can be aligned in the horizontal diffractometer plane, providing optimum resolution and sample illumination. All equations are given in closed form. The different modes of operation are compared and operation schemes are discussed.

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.

Generic nano-imprint process for fabrication of nanowire arrays

A generic process has been developed to grow nearly defect-free arrays of (heterostructured) InP and GaP nanowires. Soft nano-imprint lithography has been used to pattern gold particle arrays on full 2 inch substrates. After lift-off organic residues remain on the surface, which induce the growth of additional undesired nanowires. We show that cleaning of the samples before growth with piranha solution in combination with a thermal anneal at 550 degrees C for InP and 700 degrees C for GaP results in uniform nanowire arrays with 1% variation in nanowire length, and without undesired extra nanowires. Our chemical cleaning procedure is applicable to other lithographic techniques such as e-beam lithography, and therefore represents a generic process.

X-ray diffraction studies of potassium dihydrogen phosphate (KDP) crystal surfaces

We have studied the surface atomic structure of KDP crystals using X-ray scattering. These crystals were grown from an aqueous solution and we have done measurements both ex situ and in situ. The ex situ measurements were performed in vacuum or in air. In order to be able to do in situ measurements, we designed and built a crystal growth chamber which is compatible with X-ray diffraction experiments. The atomic arrangement of the two naturally existing faces of KDP has been determined. Preliminary results are presented of measurements performed during growth. Furthermore, the influence of metal impurities on the atomic structure of the growing interface is examined.

The structure of Si(111)-(3×3)R30°-Ag determined by surface X-ray diffraction

The Ag-induced (√3 × √3)R30° reconstruction on Si(111) has been investigated by surface X-ray diffraction. The in-plane projected structure is found from the structure factors near zero perpendicular momentum transfer. The height of the atoms in the unit cell is determined from rod profiles. The unit cell contains three Ag atoms (saturation coverage 1 monolayer) and eight Si atoms. The Ag atoms are located below a top layer of Si atoms that form a honeycomb. Large displacements from bulk positions occur. The structure is discussed in comparison with other measurements and previously proposed models.

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.