Phosphine dissociation and diffusion on Si(001) observed at the atomic scale

A detailed atomic-resolution scanning tunneling microscopy (STM) and density functional theory study of the adsorption, dissociation, and surface diffusion of phosphine (PH(3)) on Si(001) is presented. Adsorbate coverages from approximately 0.01 monolayer to saturation are investigated, and adsorption is performed at room temperature and 120 K. It is shown that PH(3) dissociates upon adsorption to Si(001) at room temperature to produce both PH(2) + H and PH + 2H. These appear in atomic-resolution STM images as features asymmetric-about and centered-upon the dimer rows, respectively. The ratio of PH(2) to PH is a function of both dose rate and temperature, and the dissociation of PH(2) to PH occurs on a time scale of minutes at room temperature. Time-resolved in situ STM observations of these adsorbates show the surface diffusion of PH(2) adsorbates (mediated by its lone pair electrons) and the dissociation of PH(2) to PH. The surface diffusion of PH(2) results in the formation of hemihydride dimers on low-dosed Si(001) surfaces and the ordering of PH molecules along dimer rows at saturation coverages. The observations presented here have important implications for the fabrication of atomic-scale P dopant structures in Si, and the methodology is applicable to other emerging areas of nanotechnology, such as molecular electronics, where unambiguous molecular identification using STM is necessary.     

Metalorganic vapor phase epitaxy of III–V-on-silicon: Experiment and theory

The integration of III–V semiconductors with Si has been pursued for more than 25 years since it is strongly desired in various high-efficiency applications ranging from microelectronics to energy conversion. In the last decade, there have been tremendous advances in Si preparation in hydrogen-based metalorganic vapor phase epitaxy (MOVPE) environment, III–V nucleation and subsequent heteroepitaxial layer growth. Simultaneously, MOVPE itself took off in its triumphal course in solid state lighting production demonstrating its power as industrially relevant growth technique. Here, we review the recent progress in MOVPE growth of III–V-on-silicon heterostructures, preparation of the involved interfaces and fabrication of devices structures. We focus on a broad range of in situ, in system and ex situ characterization techniques. We highlight important contributions of density functional theory and kinetic growth simulations to a deeper understanding of growth phenomena and support of the experimental analysis. Besides new device concepts for planar heterostructures and the specific challenges of (001) interfaces, we also cover nano-dimensioned III–V structures, which are preferentially prepared on (111) surfaces and which emerged as veritable candidates for future optoelectronic devices.     

Crystal and molecular structure of bis[(1,2-dihapto-3,5-dimethyl-pyrazolido)-π-allylpalladium(II)]

The crystal and molecular structure of bis[(1,2-dihapto-3,5-dimethylpyrazolido)--allylpalladium(II)], C₁₆H₂₄N₄Pd₂, has been determined by X-ray diffraction techniques. The crystals are monoclinic,P2₁/c (No. 14), witha = 8.752(1),b = 18.932(2),c = 11.780(3) Å, and = 109.78(1) °. The observed crystal density (1.746 g cm^–3) agrees well with that calculated on the basis of four molecules per unit cell (1.754 g cm^–3). The structure has been refined by full-matrix least-squares techniques to a finalR ₁ value of 0.045 (R ₂ = 0.056) for 2974 unique reflections withI _net7 counts sec^–1. The Pd₂N₄ ring is in a boat conformation in which the two palladium atoms are 1.113 and 1.064 Å out of the plane defined by the four nitrogen atoms. The Pd-C bond distances to the terminal atoms of the -allyl groups (2.12 Å) are longer than those to the central carbon atoms (2.06 Å). The molecule possesses approximatemm2 (C _2v ) symmetry.     

Click chemistry as a macrocyclization tool in the solid-phase synthesis of small cyclic peptides

Despite the vast number of techniques developed for the cyclization of small peptides, cyclization efficiency remains problematic in peptides that lack turn-promoting structures. Here we demonstrate the utility of click chemistry as a macrocyclization tool in the solid-phase synthesis of cyclic tetra-, penta-, hexa-, and heptapeptides. On-resin cyclization is completed at room temperature within 6 h, resulting in predominantly monomer with small amounts of cyclomultimer byproducts.     

Structural characterization of an enantiomerically pure amino acid imidazolide and direct formation of the beta-lactam nucleus from an alpha-amino acid

Decomposition of a diazo beta-ketoamide derived from N-trityl serine imidazolide and N-protected acetanilides provides, instead of the expected 3-acyloxindole product, an enantiomerically pure (EP) beta-lactam. The amino acid stereocenter is incorporated, the second chiral center is induced, and trityl protection of the beta-lactam ring is realized for the first time. The desired 3-acyloxindole is obtained from oxindole and Tr-Ser(OBn)-imidazole, the X-ray of which provides the first structural determination of an EP amino acid imidazolide.     

Chromatographic peak deconvolution of constitutional isomers by multiple-reaction-monitoring mass spectrometry

Highly efficient and sophisticated separation techniques are available to analyze complex compound mixtures with superior sensitivities and selectivities often enhanced by a 2nd dimension, e.g. a separation technique or spectroscopic and spectrometric techniques. For enantioselective separations numerous chiral stationary phases (CSPs) exist to cover a broad range of chiral compounds. Despite these advances enantioselective separations can become very challenging for mixtures of stereolabile constitutional isomers, because the on-column interconversion can lead to completely overlapping peak profiles. Typically, multidimensional separation techniques, e.g. multidimensional GC (MDGC), using an achiral 1st separation dimension and transferring selected analytes to a chiral 2nd separation are the method of choice to approach such problems. However, this procedure is very time consuming and only predefined sections of peaks can be transferred by column switching to the second dimension. Here we demonstrate for stereolabile 1,2-dialkylated diaziridines a technique to experimentally deconvolute overlapping gas chromatographic elution profiles of constitutional isomers based on multiple-reaction-monitoring MS (MRM-MS). The here presented technique takes advantage of different fragmentation probabilities and pathways to isolate the elution profile of configurational isomers.     

Crystal and Molecular Structures of Tris(1-Methylimidazol-2-yl)Phosphine,Tris(4-Methylthiazol-2-yl)Phosphine and its Sulfide

Abstract  Two crystal structures of tris(azolyl)phosphines, PR₃ [R′ = 1-methylimidazol-2-yl (1) or R′′ = 4-methylthiazol-2-yl (2)], and the crystal structure of the sulfurisation product of 2, R′′₃PS (3), were determined. All compounds crystallise in polar space groups, 1 in the orthorhombic space group Pna2₁ with cell parameters a = 13.9779(15) ?, b = 9.2492(10) ? and c = 10.2439(11) ?; 2 in the trigonal space group, R3c, with a = 15.2383(10) ? and c = 10.5882(13) ? and 3 in the orthorhombic space group, Cmc2₁, with a = 13.466(3) ?, b = 9.308(2) ? and c = 12.207(3) ?. Graphical Abstract  This article presents two crystal structures of tris(azolyl)phosphines and one of a tris(thiazolyl)phosphine sulfide, a compound which show potential for wide application in coordination chemistry due to their multidentate nature.