A three-dimensional synthetic metallic crystal composed of single-component molecules

Molecular metals normally require charge transfer between two different chemical species. We prepared crystals of [Ni(tmdt)2] (tmdt, trimethylenetetrathiafulvalenedithiolate) and carried out crystal structure analyses and resistivity measurements. The analyses and measurements revealed that these single-component molecular crystals are metallic from room temperature down to 0.6 kelvin. Ab initio molecular orbital calculations suggested that pi molecular orbitals form conduction bands. The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt)2] is a three-dimensional synthetic metal composed of planar molecules.     

Spontaneous ferroelectric order in a bent-core smectic liquid crystal of fluid orthorhombic layers

Macroscopic polarization density, characteristic of ferroelectric phases, is stabilized by dipolar intermolecular interactions. These are weakened as materials become more fluid and of higher symmetry, limiting ferroelectricity to crystals and to smectic liquid crystal stackings of fluid layers. We report the SmAP(F), the smectic of fluid polar orthorhombic layers that order into a three-dimensional ferroelectric state, the highest-symmetry layered ferroelectric possible and the highest-symmetry ferroelectric material found to date. Its bent-core molecular design employs a single flexible tail that stabilizes layers with untilted molecules and in-plane polar ordering, evident in monolayer-thick freely suspended films. Electro-optic response reveals the three-dimensional orthorhombic ferroelectric structure, stabilized by silane molecular terminations that promote parallel alignment of the molecular dipoles in adjacent layers.     

Charge Donation by Calcium into the t1g Band of C60

Photoemission spectra of compounds prepared by the reaction of C(60) films with calcium show two distinct metallic phases, whereas alkali-doped C(60) films have only one. In the first phase the bulk t(1u) band, derived from the lowest unoccupied molecular orbital of C(60), is partially occupied. This is followed by an insulating phase that has the composition Ca(3)C(60) in which the t(1u) band is filled and has properties analogous to those of K(6)C(60). Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t(1g) band. The superconductivity of Ca(5)C(60) is associated with the t(1g) band.     

退火对掺铟氧化锌薄膜结构及光学性能的影响

采用溶胶-凝胶法在石英衬底上制备掺铟氧化锌薄膜,研究不同退火温度对薄膜结构及发光性能的影响.结果表明,掺铟氧化锌薄膜仍为六角纤锌矿结构的ZnO相,在大于450 nm的波段薄膜样品的透射率都较高;随着退火温度的升高,透射率先增后减,600℃时达到最大;薄膜样品的光学带隙都小于纯ZnO的理论值(3.37 eV),且随退火温度的升高呈先减后增趋势;样品的结晶度与发光强度随着退火温度的升高而增强.     

Unexpected square symmetry seen by atomic force microscopy in bilayer films of disk-like molecules

Thin films of disk-shaped molecules are expected to display anisotropic optical and transport properties, leading to applications in optical display or sensor technologies. Bilayer Langmuir-Blodgett films of monomeric triphenylene mesogens have been studied by atomic force microscopy. The triphenylene cores of the constituent molecules tend to promote the formation of columnar structures in the plane of the substrate and along the direction of deposition of the film. Atomic force microscopy images of bilayer Langmuir-Blodgett films revealed two types of structure, one corresponding to an aligned columnar structure and the other to an unusual square lattice, which may result from the superposition of columnar structures in adjacent layers that intersect at near right angles. Annealing such bilayers near the melting point of the bulk compound improved the structural ordering by reducing the angular spread of orientations associated with the well-developed columnar structure in some areas and by producing a more distinct square lattice in other areas of the sample.     

Controlling the electronic structure of bilayer graphene

We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.     

Molecular positional order in langmuir-blodgett films by atomic force microscopy

Langmuir-Blodgett films of barium arachidate have been studied on both macroscopic and microscopic scales by atomic force microscopy. As prepared, the films exhibit a disordered hexagonal structure; molecularly resolved images in direct space establish a connection between the extent of the positional order and the presence of defects such as dislocations. Upon heating, the films reorganize into a more condensed state with a centered rectangular crystallographic arrangement; in this new state the films exhibit long-range positional order and unusual structural features, such as a height modulation of the arachidic acid molecules.     

Asymmetric scattering of polarized electrons by organized organic films of chiral molecules

Electron transmission experiments demonstrate a large asymmetry in the scattering probability of polarized electrons by thin organized films of chiral molecules. This large asymmetry results from the interaction of the electron's wavefunction with many scatterers (molecules) in the organized monolayer structure and represents a manifestation of quantum interference on the scale of supramolecular lengths.     

Coherent electronic fringe structure in incommensurate silver-silicon quantum wells

Atomically uniform silver films grown on highly doped n-type Si(111) substrates show fine-structured electronic fringes near the silicon valence band edge as observed by angle-resolved photoemission. No such fringes are observed for silver films grown on lightly doped n-type substrates or p-type substrates, although all cases exhibited the usual quantum-well states corresponding to electron confinement in the film. The fringes correspond to electronic states extending over the silver film as a quantum well and reaching into the silicon substrate as a quantum slope, with the two parts coherently coupled through an incommensurate interface structure.     

Formation of a one-dimensional array of oxygen in a microporous metal-organic solid

We report the direct observation of dioxygen molecules physisorbed in the nanochannels of a microporous copper coordination polymer by the MEM (maximum entropy method)/Rietveld method, using in situ high-resolution synchrotron x-ray powder diffraction measurements. The obtained MEM electron density revealed that van der Waals dimers of physisorbed O2 locate in the middle of nanochannels and form a one-dimensional ladder structure aligned to the host channel structure. The observed O-O stretching Raman band and magnetic susceptibilities are characteristic of the confined O2 molecules in one-dimensional nanochannels of CPL-1 (coordination polymer 1 with pillared layer structure).     

Strong enhancement of nonlinear optical properties through supramolecular chirality

A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of the molecules. The chiral supramolecular organization makes the second-order NLO susceptibility about 30 times larger for the nonracemic material than for the racemic material with the same chemical structure. The susceptibility of the nonracemic films is a respectable 50 picometers per volt, even though the helicene structure lacks features commonly associated with high nonlinearity. Susceptibility components that are allowed only by chirality dominate the second-order NLO response.     

Divalent metal nanoparticles

Nanoparticles can be used as the building blocks for materials such as supracrystals or ionic liquids. However, they lack the ability to bond along specific directions as atoms and molecules do. We report a simple method to place target molecules specifically at two diametrically opposed positions in the molecular coating of metal nanoparticles. The approach is based on the functionalization of the polar singularities that must form when a curved surface is coated with ordered monolayers, such as a phase-separated mixture of ligands. The molecules placed at these polar defects have been used as chemical handles to form nanoparticle chains that in turn can generate self-standing films.     

热加工过程淀粉基膜材结构变化和增塑剂迁移研究

[目的]研究不同加热方式下,淀粉基膜材的结构变化和增塑剂迁移情况,为新型淀粉基膜材在食品包装中的应用提供参考。[方法]以蒸馏水体系为热加工介质,采用沸水加热和微波加热,对淀粉基膜材进行热加工处理并分析加热过程中膜材表层增塑剂迁移、总迁移及膜材基体质量和结构变化。[结果]沸水加热和微波加热均加剧了淀粉基膜材表面增塑剂的迁移,淀粉高分子片段溶出,膜材内部产生高分子聚集,促使膜材总体迁移增大;当增塑剂含量较低时,2种热加工处理后的膜材断面形成了更加明显的孔洞结构。另外,相比于沸水加热,微波加热较高的瞬时功率促使膜材表层增塑剂迁移、总迁移程度更大。[结论]不同的热传递模式导致淀粉基膜材结构变化与分子迁移存在差异。     

A bond-fluctuation mechanism for stochastic switching in wired molecules

Stochastic on-off conductivity switching observed in phenylene-ethynylene oligomers has been explained in terms of changes in ring conformations, or electron localization, or both. We report the observation of stochastic on-off switching in the simplest of wired molecules: octanedithiol, decanedithiol, and dodecanedithiol bonded on an Au(111) surface. Stochastic switching was observed even when a top gold contact was pressed on by a conducting atomic force microscope tip at constant force. The rate of switching increased substantially at 60 degrees C, a temperature at which these films are commonly annealed. Because such switching in alkanethiols is unlikely to be caused by internal molecular electronic changes and cannot be fully accounted for by breaking of the top contact, we argue that the cause is the well-known mobility of molecules tethered to gold via a thiol linkage.     

Crystal structure of the 1: 1 complex of 5-fluorouracil and 9-ethylhypoxanthine

9-Ethylhypoxanthine and 5-fluorouracil form a 1 : 1 crystalline complex. The structure of this complex has been solved by x-ray diffraction analysis. The molecules crystallize in a monoclinic lattice and form a sheet structure in which pairs of fluorouracil molecules are held together by two hydrogen bonds. The 9-ethylhypoxanthine residues fill up the rest of the molecular sheet by forming single hydrogen bonds with each uracil pair.     

Optical pumping and vibrational cooling of molecules

The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels nu of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the nu = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation ( approximately 70% of the initially detected molecules) in the lowest vibrational level, nu = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.     

The spectroscopy of very cold gases

The technique of supersonic free jet spectroscopy can be used to study the structure and dynamics of molecules which have been cooled to far below their boiling points but which remain in the gas phase. Cooling of the internal degress of freedom, the molecular rotations and vibrations, produces a highly resolved and greatly simplified molecular spectrum. The principles of the technique are discussed and its utility is demonstrated by two examples. the spectroscopy of porphyrins in the gas phase and the photochemistry of van der Waals molecules.     

Dispersible exfoliated zeolite nanosheets and their application as a selective membrane

Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.     

分子激子理论及其在光谱解析中的应用

分子聚集体是通过分子间相互作用形成的,其特殊作用和功能首先在生物体系中得到了人们的认识。分子光谱是聚集态分子研究的有力工具。分子激子理论基于激发态共振相互作用处理分子聚集体,给出了分子聚集体的光谱性质与分子结构之间的关系。本文将对分子激子理论做一简要介绍,并给出分子激子理论在光谱解析中的几个例子。     

The structure of the first coordination shell in liquid water

X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen-bonded tetrahedral structure in ice. Upon heating from 25 degrees C to 90 degrees C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen-bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed.