Van Der Waals Epitaxial Growth of Bismuth Thin-film on Silicon (111) Substrate by MBE

Crystallinity of an 80-nm-thick bismuth thin film grown on Si(111) substrate by MBE was investigated. The highly (0003) textured Bi film contains two twinning domains with different bilayer stacking sequences. The basic lattice parameters c and a as well as b, the bilayer thickness, of the two domains were determined from a series of X-ray diffraction (XRD) measurements, and found that the differences are within 0.1% as compared with those of bulk Bi reported in literature, suggesting that the Bi film has been nearly fully relaxed. From the XRD φ-scans of asymmetric Bi (01-14), (10-15), (11-26) planes and Si (220) plane as well as selected area electron diffraction patterns and electron back scatter diffraction pole figures, we confirmed the well registration between the lattices of Si and Bi lattice, i.e. the ω angle difference between Bi[0003] and Si[111] and the φ angle different between Bi[01-14] and Si[220] are 0.056° and 0.25°, respectively, and thus concluded that the growth is a quasi-van der Waals epitaxy.

Numerical Simulation of Bubble Transport in a Bifurcating Microchannel: A Preliminary Study

In this paper, we present the computational fluid dynamics (CFD) simulations of bubble transport in a first generation bifurcating microchannel. In the present study, the human arteriole is modeled as a two-dimensional (2D) rectangular bifurcating microchannel. The microchannel is filled with blood and a single perfluorocarbon (PFC) bubble is introduced in the parent channel. The simulations are carried out to identify the lodging and dislodging pressures for two nondimensional bubble sizes, Ld (ratio of the dimensional bubble length to the parent tube diameter), that is for Ld = 1 and Ld = 2. Subsequently, the bubble transport and splitting behavior due to the presence of symmetry and asymmetry in the daughter channels of the microchannel is studied for these bubble sizes. The splitting behavior of the bubble under the effect of gravity is also assessed and reported here. For the symmetric bifurcation model, the splitting ratio (SR) (ratio of bubble volume in bottom daughter channel to bubble volume in top daughter channel), of the bubble was found to be 1. For the asymmetric model, the splitting ratio was found to be less than 1. The loss in the bubble volume in the asymmetric model was attributed to surface tension effects and the resistance offered by the flow, which led to the bubble sticking and sliding along the walls of the channel. With the increase in roll angle, Φ (angle which the plane makes with the horizontal to study the effects of gravity), there was a decline in the splitting ratio.

Chiroptical properties and conformation of N-acetyl-L-amino acids N'-methylamides with aliphatic side chains

CD and 1H NMR spectra of L-alanine (I), L-leucine (II), L-valine (III) and L-tert-leucine (IV) N-acetyl N'-methylamides were measured at various experimental conditions involving changes of temperature, concentration and solvent polarity. The least flexible tert-leucine derivative IV exists predominantly either in right handed α-helical (αR) conformation (fluorinated alcohols, ethanol) or in extended (C5) conformation (cyclohexane, acetonitrile). In this compound the Φ angle is constrained to about -120°, excepting aqueous solution. The flexible amides I and II exhibit more complex conformational equilibria involving probably the 310, αR and C5 conformations. The 310 helical conformation is favoured in water, acetonitrile and non-fluorinated alcohols, while a higher participation of αR conformation is observed in strongly polar fluorinated alcohols. Conformational distribution of the valine derivatve III is similar to IV in fluorinated alcohols and to amides I and II in other solvents. The C7eq conformation is clearly detected only in cyclohexane solutions of non-associated forms of I and II.

Interaction and flexibility of molecules in hydrocarbon aggregates, torsional potential in a group of hexanes

The molecular-mechanics method has been used for calculation of the torsional potential V(φ) of internal rotation around central C-C bond in hexane with one or two other hexane molecules in the neighbourhood and with full optimization of external and internal degrees of freedom. The optimum energy arrangement of the hydrocarbon molecules has been determined for various values of the φ angle as well as the optimum intramolecular geometry parameters. The adjacent molecules with all-trans conformation accommodate to the shape variation of the rotating molecule by means of translation motion. The adjacent molecules can change substantially the form of V(φ) potential as compared with the isolated molecule, and they increase rigidity of the hydrocarbon chain. For some types of mutual arrangement, however, it has been found that the other molecule in the vicinity decrease the conformational energies and barriers of transitions between the rotation isomers as compared with the isolated molecule. Thus the inter-chain forces can induce also an effective coiling of the chain. Implications of these findings have been discussed with respect to structural properties of various real hydrocarbon aggregates in solution and in solid state.