Experimental and theoretical analysis of hydrogen bonding in two-dimensional chiral 4′,4′′′′-(1,4-Phenylene)bis(2,2′:6′,2″-terpyridine) self-assembled nanoarchitecture

ABSTRACT: The two-dimensional self-assembly of 4′,4⁗- (1,4-phenylene)bis(2,2′:6′,2″-terpyridine) molecules is exper- imentally and theoretically investigated. Scanning tunneling microscopy (STM) shows that this molecular building block forms a compact chiral supramolecular network on graphite at the 1-octanol/graphite interface. The molecules adopt a side- by-side arrangement inside the organic domains. In contrast, the molecules are arranged perpendicularly at the domain boundary. Detailed theoretical analysis based on the density functional theory (DFT) shows that these arrangements are stabilized by double and single hydrogen bonds between pyridine groups. Only the molecular peripheral pyridine groups are involved in the hydrogen bonds stabilizing the long-range ordered molecular nanoarchitectures

Engineering porous and compact two-dimensional nanoarchitectures on surfaces taking advantage of bisterpyridine-derivatives self-assembly

International audienceThe self-assembly of two bis-terpyridine derivatives is experimentally investigated at the nanometer scale. Scanning tunneling microscopy (STM) reveals that two-dimensional compact and porous nanoarchitectures can be engineered by changing the length of terpyridine spacer; i.e. a benzene ring or a quaterthiophene (4T) unit. In both cases the molecular nanoarchitecture appears to be stabilized by double hydrogen-bonds between molecular terpyridine groups. The STM images suggest however that terpyridine groups adopt different conformations, s-cis and s-trans as well as s-trans and s-trans conformations, in the two self-assembled organic layers

Out- versus in-plane magnetic anisotropy of free Fe and Co nanocrystals: tight-binding and first-principles studies

We report tight-binding (TB) and Density Function Theory (DFT) calculations of magnetocrystalline anisotropy energy (MAE) of free Fe (body centerd cubic) and Co (face centered cubic) slabs and nanocrystals. The nanocrystals are truncated square pyramids which can be obtained experimentally by deposition of metal on a SrTiO$_3$(001) substrate. For both elements our local analysis shows that the total MAE of the nanocrystals is largely dominated by the contribution of (001) facets. However, while the easy axis of Fe(001) is out-of-plane, it is in-plane for Co(001). This has direct consequences on the magnetic reversal mechanism of the nanocrystals. Indeed, the very high uniaxial anisotropy of Fe nanocrystals makes them a much better potential candidate for magnetic storage devices.Comment: 8 pages, 7 figure

Fabrication of a Complex Two-Dimensional Adenine Perylene-3,4,9,10-tetracarboxylic Dianhydride Chiral Nanoarchitecture through Molecular Self-Assembly

International audienceThe two-dimensional self-assembly of a nonsymmetric adenine DNA base mixed with symmetric perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules is investigated using scanning tunneling microscopy (STM). We experimentally observe that these two building blocks form a complex close-packed chiral supramolecular network on Au(111). The unit cell of the adenine PTCDA nanoarchitecture is composed of 14 molecules. The high stability of this structure relies on PTCDA PTCDA and PTCDA adenine hydrogen bonding. Detailed theoretical analysis based on the density functional theory (DFT) calculations reveals that adenine molecules work as a "glue", providing additional strengthening to the PTCDA-based skeleton of this sophisticated multicomponent nanoarchitecture. At the same time, we find that orientation and chirality of adenine molecules across the monolayer is likely to vary, leading to a disorder in the atomistic structure of the entire assembly

Two-Dimensional 1,3,5-Tris(4-carboxyphenyl)benzene Self-Assembly at the 1-Phenyloctane/Graphite Interface Revisited

International audienceTwo-dimensional (2D) self-assembly of star-shaped 1,3,5-tris(4-carboxyphenyl)benzene molecules is investigated. Scanning tunneling microscopy reveals that this molecule can form three hydrogen-bonded networks at the 1-phenyloctane/graphite interface. One of these structures is close-packed and the two other ones are porous structures, with hexagonal and rectangular cavities. The network with rectangular cavities appears to be the most stable structure

Engineered inorganic core/shell nanoparticles

International audienceIt has been for a long time recognized that nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic structures. At first, size effects occurring in single elements have been studied. More recently, progress in chemical and physical synthesis routes permitted the preparation of more complex structures. Such structures take advantages of new adjustable parameters including stoichiometry, chemical ordering, shape and segregation opening new fields with tailored materials for biology, mechanics, optics magnetism, chemistry catalysis, solar cells and microelectronics. Among them, core/shell structures are a particular class of nanoparticles made with an inorganic core and one or several inorganic shell layer(s). In earlier work, the shell was merely used as a protective coating for the core. More recently, it has been shown that it is possible to tune the physical properties in a larger range than that of each material taken separately. The goal of the present review is to discuss the basic properties of the different types of core/shell nanoparticles including a large variety of heterostructures. We restrict ourselves on all inorganic (on inorganic/inorganic) core/shell structures. In the light of recent developments, the applications of inorganic core/shell particles are found in many fields including biology, chemistry, physics and engineering. In addition to a representative overview of the properties, general concepts based on solid state physics are considered for material selection and for identifying criteria linking the core/shell structure and its resulting properties. Chemical and physical routes for the synthesis and specific methods for the study of core/shell nanoparticle are briefly discussed

The Small Unit Cell Reconstructions of SrTiO3 (111)

We analyze the basic structural units of simple reconstructions of the (111) surface of SrTiO3 using density functional calculations. The prime focus is to answer three questions: what is the most appropriate functional to use; how accurate are the energies; what are the dominant low-energy structures and where do they lie on the surface phase diagram. Using test calculations of representative small molecules we compare conventional GGA with higher-order methods such as the TPSS meta-GGA and on-site hybrid methods PBE0 and TPSSh, the later being the most accurate. There are large effects due to reduction of the metal d oxygen sp hybridization when using the hybrid methods which are equivalent to a dynamical GGA+U, which leads to rather substantial improvements in the atomization energies of simple calibration molecules, even though the d-electron density for titanium compounds is rather small. By comparing the errors of the different methods we are able to generate an estimate of the theoretical error, which is about 0.25eV per 1x1 unit cell, with changes of 0.5-1.0 eV per 1x1 cell with the more accurate method relative to conventional GGA. An analysis of the plausible structures reveals an unusual low-energy TiO2-rich configuration with an unexpected distorted trigonal biprismatic structure. This structure can act as a template for layers of either TiO or Ti2O3, consistent with experimental results as well as, in principle, Magnelli phases. The results also suggest that both the fracture surface and the stoichiometric SrTiO3 (111) surface should spontaneously disproportionate into SrO and TiO2 rich domains, and show that there are still surprises to be found for polar oxide surfaces.Comment: 14 pages, 4 Figure