Controlling the dynamics of a single atom in lateral atom manipulation

We studied the dynamics of a single cobalt (Co) atom during lateral manipulation on a copper (111) surface in a low-temperature scanning tunneling microscope. The Co binding site locations were revealed in a detailed image that resulted from lateral Co atom motion within the trapping potential of the scanning tip. Random telegraph noise, corresponding to the Co atom switching between hexagonal close-packed (hcp) and face-centered cubic (fcc) sites, was seen when the tip was used to try to position the Co atom over the higher energy hcp site. Varying the probe tip height modified the normal copper (111) potential landscape and allowed the residence time of the Co atom in these sites to be varied. At low tunneling voltages (less than approximately 5 millielectron volts), the transfer rate between sites was independent of tunneling voltage, current, and temperature. At higher voltages, the transfer rate exhibited a strong dependence on tunneling voltage, indicative of vibrational heating by inelastic electron scattering.     

Orientational dielectric relaxation of collisionless molecules

The generation of the orientation component of the polarization of matter in an electric field has previously been thought to require interaction of molecules with their neighbors. It is demonstrated that, even in the absence of collisions between neighboring molecules, hot isolated polyatomic molecules can reorient in response to an external field, thereby giving rise to the orientation component of polarization. This reorientation occurs through the interaction of rotation with molecular vibrations, which provides a heat bath to establish thermal rotational equilibrium. This effect is demonstrated for o-difluorobenzene, o-dichlorobenzene, and p-chlorotoluene, with an inhomogeneous electric field used to deflect molecular beams of these molecules.     

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.     

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.     

Reversible optical transcription of supramolecular chirality into molecular chirality

In nature, key molecular processes such as communication, replication, and enzyme catalysis all rely on a delicate balance between molecular and supramolecular chirality. Here we report the design, synthesis, and operation of a reversible, photoresponsive, self-assembling molecular system in which molecular and supramolecular chirality communicate. It shows exceptional stereoselectivity upon aggregation of the molecules during gel formation with the solvent. This chirality is locked by photochemical switching, a process that is subsequently used to induce an inverted chiral supramolecular assembly as revealed by circular dichroism spectroscopy. The optical switching between different chiral aggregated states and the interplay of molecular and supramolecular chirality offer attractive new prospects for the development of molecular memory systems and smart functional materials.     

Probing the chiroptical response of a single molecule

Chirally sensitive measurement techniques have generally been restricted to bulk samples. Here, we report the observation of fluorescence-detected circular dichroism (FDCD) from single (bridgedtriarylamine) helicene molecules by using an excitation wavelength (457 nanometers) in the vicinity of an electronic transition that shows circular dichroism in bulk samples. The distributions of dissymmetry (g) parameters by analysis of signals from pure M- and P-type diastereomers are almost perfect mirror images of one another, each spanning a range of both positive and negative values. In addition, we observe a well-defined structure in the histogram of dissymmetry parameters suggestive of specific molecular orientations at the polymer interface. These single-molecule results highlight strong intrinsic circular dichroism responses that can be obscured by cancellation effects in ensemble measurements of a randomly oriented bulk sample.     

Organic transistors: two-dimensional transport and improved electrical characteristics

The thiophene oligomer alpha-hexathienylene (alpha-6T) has been successfully used as the active semiconducting material in thin-film transistors. Field-induced conductivity in thin-film transistors with alpha-6T active layers occurs only near the interfacial plane, whereas the residual conductivity caused by unintentional doping scales with the thickness of the layer. The two-dimensional nature of the field-induced conductivity is due not to any anisotropy in transport with respect to any molecular axis but to interface effects. Optimized methods of device fabrication have resulted in high field-effect mobilities and on/off current ratios of > 10(6). The current densities and switching speeds are good enough to allow consideration of these devices in practical large-area electronic circuits.     

一氧化氮在植物抗逆境信号转导中的作用

概述了NO分子在植物体内的来源,在信号转导、生物和非生物胁迫响应等过程中作用,以及NO与植物中其他信号分子的关系。     

Illuminating single molecules in condensed matter

Efficient collection and detection of fluorescence coupled with careful minimization of background from impurities and Raman scattering now enable routine optical microscopy and study of single molecules in complex condensed matter environments. This ultimate method for unraveling ensemble averages leads to the observation of new effects and to direct measurements of stochastic fluctuations. Experiments at cryogenic temperatures open new directions in molecular spectroscopy, quantum optics, and solid-state dynamics. Room-temperature investigations apply several techniques (polarization microscopy, single-molecule imaging, emission time dependence, energy transfer, lifetime studies, and the like) to a growing array of biophysical problems where new insight may be gained from direct observations of hidden static and dynamic inhomogeneity.     

Current-induced hydrogen tautomerization and conductance switching of naphthalocyanine molecules

The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by low-temperature scanning tunneling microscopy. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by resonant tunneling through the molecule and is expressed as considerable changes in the conductivity of the molecule. We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.     

Guest-dependent spin crossover in a nanoporous molecular framework material

The nanoporous metal-organic framework Fe2(azpy)4(NCS)4.(guest) (azpy is trans-4,4'-azopyridine) displays reversible uptake and release of guest molecules and contains electronic switching centers that are sensitive to the nature of the sorbed guests. The switching of this material arises from the presence of iron(II) spin crossover centers within the framework lattice, the sorbed phases undergoing "half-spin" crossovers, and the desorbed phase showing no switching property. The interpenetrated framework structure displays a considerable flexibility with guest uptake and release, causing substantial changes in the local geometry of the iron(II) centers. The generation of a host lattice that interacts with exchangeable guest species in a switchable fashion has implications for the generation of previously undeveloped advanced materials with applications in areas such as molecular sensing.     

Friction anisotropy and asymmetry of a compliant monolayer induced by a small molecular tilt

Lateral force microscopy in the wearless regime was used to study the friction behavior of a lipid monolayer on mica. In the monolayer, condensed domains with long-range orientational order of the lipid molecules were present. The domains revealed unexpectedly strong friction anisotropies and non-negligible friction asymmetries. The angular dependency of these effects correlated well with the tilt direction of the alkyl chains of the monolayer, as determined by electron diffraction and Brewster angle microscopy. The molecular tilt causing these frictional effects was less than 15 degrees, demonstrating that even small molecular tilts can make a major contribution to friction.     

Water at hydrophobic surfaces: weak hydrogen bonding and strong orientation effects

Vibrational studies that selectively probe molecular structure at CCl4/H2O and hydrocarbon/H2O interfaces show that the hydrogen bonding between adjacent water molecules at these interfaces is weak, in contrast to generally accepted models of water next to fluid hydrophobic surfaces that suggest strong hydrogen bonding. However, interactions between these water molecules and the organic phase result in substantial orientation of these weakly hydrogen-bonded water molecules in the interfacial region. The results have important implications for understanding water adjacent to hydrophobic surfaces and the penetration of water into hydrophobic phases.     

NaCl对杉木中水分子扩散影响的机理研究

【目的】NaCl的存在会改变木材中水分子的扩散行为,使水分子"易进入"或"难扩散",从而影响了木材的吸湿性和平衡含水率。研究其影响机理对含盐木质文物的保护具有重要意义。【方法】以杉木为研究对象,制作了不同含盐率的杉木试件,开展吸湿与干燥试验,探究盐分对杉木中水分扩散的宏观影响规律;建立了杉木的分子模型,采用分子动力学(MD)模拟方法分别研究了水分子进入含盐与不含盐两种模型的过程,以及水分子在两种模型中的扩散行为。通过平衡构型、相对浓度分布等参数分析水分子进入两种模型结果的差异;通过均方位移、吸附能、径向分布函数、氢键等参数分析两种模型中的水分子与纤维素分子、半纤维素分子、木质素分子等杉木主要组成成分之间的相互作用关系,定性解释了盐分影响杉木中水分扩散的微观机理。【结果】试验结果表明杉木试件的吸湿速率随着含盐率的升高而增大;在含水率低于80%范围内,干燥速率随含盐率的升高而减小。MD模拟结果表明,水分子更容易进入含有NaCl的杉木分子模型中,但NaCl会抑制水分子在杉木分子模型中的扩散。模型中各分子之间的相互作用分析结果显示,NaCl与水分子间有很强的吸附作用,但会降低水分子与杉木之间作...     

Real-space observation of molecular motion induced by femtosecond laser pulses

Femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu110 surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in situ scanning tunneling microscopy. Adsorbate motion both along and perpendicular to the rows of the Cu110 surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. The experimental findings for the probability and direction of the molecular motion can be understood as a manifestation of strong coupling between the adsorbates' lateral degrees of freedom and the substrate electronic excitation produced by the femtosecond laser radiation.     

Chemistry. How to assemble a molecular junction

Junctions between metals and molecules play an important role in molecular electronics. Advances in this field are hampered by the lack of understanding of the electronic structure of organic-metal interfaces. In his Perspective, Kummel highlights the report by Nazin et al. (3), who have used scanning tunneling microscopy (STM) to assemble a metal-molecule-metal junction. Subsequently, they employed the STM tip to probe the atomic structure and local electronic properties of the metal-molecule interface in unprecedented detail. They find evidence for strong coupling between the molecular and metal states. Such coupling affects the conductivity of metal-molecule-metal junctions.     

High harmonic generation from multiple orbitals in N2

Molecular electronic states energetically below the highest occupied molecular orbital (HOMO) should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival. This feature indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HOMO-1. Such observations of lower-lying orbitals are essential to understanding subfemtosecond/subangstrom electronic motion in laser-excited molecules.     

Electrostatic aspects of physical adsorption: implications for molecular sieves and gaseous anesthesia

The equations of electrostatics are applied to the adsorption of gases on molecular sieves; separations on sieves are caused not by the size of molecules but by electrostatic forces between the gases and the strong electric fields of the sieves. Electrostatic interactions can also explain the phenomenon of general anesthesia.     

Mechanism of polarized light perception

As background for a report on our current selective adaptation experiments in decapod crustaceans, the various facts and hypotheses generally relevant to intraretinal sensitivity to polarized light in arthropods as well as cephalopods have been marshaled. On the basis of this review, the following working hypotheses have been made. 1) One ommatidium in the compound eye is the functional unit in image perception but contains in its component retinular cells subunits which can work independently in detecting other visual parameters, such as polarization. 2) Single retinular cells do respond differentially to light polarized in various planes. 3) Light sensitivity, including e-vector detection, is localized in the rhab domeres, which comprise closely packed arrays of microvilli protruding axially from retinular cells; the dichroism of the photopigment molecules, which are contained within the microvilli, provides the molecular basis of e-vector detection. 4) The visual pigment molecules have their major dichroic axis aligned predominantly parallel to the long axis of the microvillus containing them; typically all microvilli in a single rhab domere are closely parallel to one another, thus comprising at the cellular level a unit dichroic analyzer with maximum optical density to photons vibrating in the direction parallel to these microvillous protrusions. 5) In most decapod crustaceans, in cephalopods, and in some insects the microvilli in all rhabdomeres of a retinula are oriented in only two directions, perpendicular. to each other. Therefore, e-vector perception must depend at the retinular level on a two channel system consisting of a pair of dichroic analyzers with their major transmitting axes fixed at a 90 degrees angle determined by the two directions of microvillus orientation. Our new results on selective adaptation in the eye of Cardisoma provide direct experimental evidence for such a two-channel analyzer in which the pair of functional units have their maximum sensitivity to polarization in the same retinal directions as the rhab dom microvilli observed in electron micrographs. In turn, these directions correspond with the vertical and horizontal axes of the animal's normal spatial orientation. In e-vector detection the seven retinular cells of a single decapod ommatidium thus form two operational subgroups of four and three cells, respectively (39). The correspondence of the electrophysiological evidence for a dual polarization analyzer with the perpendicular directions shown by the microvilli in a single rhabdom strengthens the idea that one ommatidium is enough for detecting e-vector orientation. On this evidence we may conclude that the model developed above for a two-channel polarization analyzer effectively accounts for the relevant spectrophotometric, fine-structural, electrophysiological, and behavioral data currently available for a considerable number of arthropods and cephalopods.     

A ferroelectric liquid crystal conglomerate composed of racemic molecules

We describe the design and synthesis of a ferroelectric liquid crystal composed of racemic molecules. The ferroelectric polarization results from spontaneous polar symmetry breaking in a fluid smectic. The ferroelectric phase is also chiral, resulting in the formation of a mixture of macroscopic domains of either handedness at the isotropic-to-liquid crystal phase transition. This smectic liquid crystal is thus a fluid conglomerate. Detailed investigation of the electrooptic and polarization current behavior within individual domains in liquid crystal cells shows the thermodynamically stable structure to be a uniformly tilted smectic bow-phase (banana phase), with all layer pairs homochiral and ferroelectric (SmC(S)P(F)).