Determination of preferential rare earth adatom adsorption geometries on Si(001)

The adsorption patterns of rare earth atoms on Si(001) were investigated using scanning tunneling microscopy measurements and density functional calculations. Stable configurations were systematically determined via calculation of binding energies of various adatom coverage and adsorption geometry. Competition between inter-adatom hybridization and Coulomb repulsion is the mechanism contributing to binding energy minima associated with commonly observed rare earth adsorption geometries. Comparison of stable configurations with experimental scanning tunneling microscopy images demonstrated accuracy of the theoretical models. This paves a way for the understanding of self-assembly of rare earth disilicide nanowires on vicinal Si(001) substrates.     

Self-assembled monolayers on Pt(111): molecular packing structure and strain effects observed by scanning tunneling microscopy

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.     

Inclusive Ξ0, Ξ(1530)- and Ξ(1530)0 production in Ξ--Be interactions at 116 GeV/c

In an experiment using the CERN SPS hyperon beam, we have measured the cross sections for inclusive production of Ξ⁰ (accompanied by a charged particle), Ξ(1530)^- and Ξ(1530)⁰ in Ξ^--beryllium collisions at 116 GeV/c. All differential cross sections are found to be well described by the from (1?x _F )ⁿ exp (?bp _T ² ) over the whole accessible kinematical region (x _F >0.3). The invariant production spectra for Ξ⁰, Ξ(1530)^- and Ξ(1530)⁰, as well as those for Ξ^-, and Ω^- (obtained previously from the same data), are compared to theoretical predictions deduced from a two-component quark-diquark cascade model.     

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H-bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H-bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue-to-orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.     

Enantioselective synthesis of nicotinic receptor probe 7,8-difluoro-1,2,3,4,5,6- hexahydro-1,5-methano-3-benzazocine

[Structure: see text] The development of a concise enantioselective synthesis of nicotinic alkaloid 1 is presented. The route features the synthesis and use of a "stable" aliphatic triflate 21 in an alkylation step to generate Heck precursor 24 and an enantioselective cyclization to establish a compound with the key [3.2.1]-bicyclic core, 29.     

High-affinity interactions of ligands at recombinant Guinea pig 5HT7 receptors

The serotonin 5HT₇ receptor has been implicated in numerous physiological and pathological processes from circadian rhythms [1] to depression and schizophrenia. Clonal cell lines heterologously expressing recombinant receptors offer good models for understanding drug-receptor interactions and development of quantitative structure-activity relationships (QSAR). Comparative Molecular Field Analysis (CoMFA) is an important modern QSAR procedure that relates the steric and electrostatic fields of a set of aligned compounds to affinity. Here, we utilized CoMFA to predict affinity for a number of high-affinity ligands at the recombinant guinea pig 5HT₇ receptor. Using R-lisuride as the template, a final CoMFA model was derived using procedures similar to those of our recent papers [2, 3, 4] The final cross-validated model accounted for >85% of the variance in the compound affinity data, while the final non-cross validated model accounted for >99% of the variance. Model evaluation was done using cross-validation methods with groups of 5 ligands. Twenty cross-validation runs yielded an average predictive r²(q²) of 0.779 ± 0.015 (range: 0.669–0.867). Furthermore, 3D-chemical database search queries derived from the model yielded hit lists of promising agents with high structural similarity to the template. Together, these results suggest a possible basis for high-affinity drug action at 5HT₇ receptors.     

Thermal expansion coefficients of rare earth metal disilicides and their influence on the growth of disilicide nanowires

The lattice parameters of rare earth metal disilicides at high temperature play a crucial role in the growth of these disilicide nanowires, which takes place at about 873 K. In the present study, the lattice parameters of Sm, Gd, Tb, Dy, Ho, Er, Sc and Er_xGd_1-x disilicides with AlB₂ structure were measured from room temperature to about 873 K by high temperature X-ray diffraction (HTXRD), from which the linear coefficient of thermal expansion (CTE) along each crystallographic axis was determined. The influence of the CTEs on the growth of nanowires will be discussed in terms of anisotropic strain. With these CTEs, the composition of a ternary disilicide, (Er_xGd_1-x)Si_1.67, for example, with optimal lattice parameters at nanowire growth temperature can be engineered to meet the requirement for the growth of high-quality nanowires. PACS 65.40.De; 68.65.La; 68.55.-a     

Phase stabilities of ternary rare earth metal disilicides

Chen et al. recently carried out self-assembled growth of epitaxial sub-10 nm erbium disilicide (C32, hP3) nanowires on Si(001). They pointed out that the success of this self-assembly process is due to asymmetric lattice mismatches in the two orthogonal crystallographic directions of the two materials, i.e. [0001]_ErSi2/[11̄0]_Si,+6.5% and [112̄0]_RESi2/[110]_Si,-1.3%. In this paper, we have established experimentally that (Er_1-xGd_x)Si₂ and (Er_1-xSm_x)Si₂ exist with values of x from 0 to 1 and that their lattice parameters follow Vegards law. Since the binary GdSi₂ and SmSi₂ end members in each of these ternary systems have a and c unit cell parameters greater than those of the ErSi₂ end member, we have determined that an optimal lattice mismatch can be achieved with the Si substrate at a composition of (Er_0.45Gd_0.55)Si₂ and (Er_0.62Sm_0.38)Si₂. It is reasonable to expect a higher quality of self-assembled nanowire growth on Si by employing these ternary silicides. A review of the atomic sizes of the rare earth metal elements of the lanthanide series indicates that many of the binary disilicides, when exhibiting the C32 structure, should have a tendency to form ternary and higher-order disilicides. These materials would offer many possibilities for the growth of silicide nanowires with interesting electronic, magnetic and optical properties. PACS 64; 61.10.Nz