Anisotropic 2D Cu2-x Se Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe2 Nanoparticles.

We present the synthesis of colloidal anisotropic Cu2-x Se nanocrystals (NCs) with excellent size and shape control, using the unexplored phosphine-free selenium precursor 1-dodecaneselenol (DDSe). This precursor forms lamellar complexes with Cu(I) that enable tailoring the NC morphology from 0D polyhedral to highly anisotropic 2D shapes. The Cu2-x Se NCs are subsequently used as templates in postsynthetic cation exchange reactions, through which they are successfully converted to CdSe and CuInSe2 quantum dots, nanoplatelets, and ultrathin nanosheets. The shape of the template hexagonal nanoplatelets is preserved during the cation exchange reaction, despite a substantial reorganization of the anionic sublattice, which leads to conversion of the tetragonal umangite crystal structure of the parent Cu2-x Se NCs into hexagonal wurtzite CdSe and CuInSe2, accompanied by a change of both the thickness and the lateral dimensions of the nanoplatelets. The crystallographic transformation and reconstruction of the product NCs are attributed to a combination of the unit cell dimensionalities of the parent and product crystal phases and an internal ripening process. This work provides novel tools for the rational design of shape-controlled colloidal anisotropic Cu2-x Se NCs, which, besides their promising optoelectronic properties, also constitute a new family of cation exchange templates for the synthesis of shape-controlled NCs of wurtzite CdSe, CuInSe2, and other metal selenides that cannot be attained through direct synthesis approaches. Moreover, the insights provided here are likely applicable also to the direct synthesis of shape-controlled NCs of other metal selenides, since DDSe may be able to form lamellar complexes with several other metals.

Combination of Scanning Probe Microscopy and Coordination Chemistry: Structural and Electronic Study of Bis(methylbenzimidazolyl)ketone and Its Iron Complex.

Here, we report the bulk synthesis of [FeII(BMBIK)Cl2] bearing the redox noninnocent bis(methylbenzimidazolyl)ketone (BMBIK) ligand and the synthesis of the similar complex [FeI(BMBIK)]+ on a Au(111) surface using lateral manipulation at the atomic level. Cyclic voltammetry and scanning tunneling spectroscopy are shown to be useful techniques to compare the coordination compound in solution with the one on the surface. The total charge, as well as the oxidation and spin state of [FeI(BMBIK)]+, are investigated by comparison of the shape of the lowest unoccupied molecular orbital (LUMO), visualized by tunneling through the LUMO, with theoretical models. The similar reduction potentials found for the solution and surface compounds indicate that the major effect of lowering the LUMO upon coordination of BMBIK to the iron center is conserved on the surface. The synthesis and analysis of [FeI(BMBIK)]+ using scanning tunneling microscopy, scanning tunneling spectroscopy, and atomic force microscopy are the first steps toward mechanistic studies of homogeneous catalysts with redox noninnocent ligands at the single molecule level.

Characteristic Contrast in Δfmin Maps of Organic Molecules Using Atomic Force Microscopy.

Scanning tunneling microscopy and atomic force microscopy can provide detailed information about the geometric and electronic structure of molecules with submolecular spatial resolution. However, an essential capability to realize the full potential of these techniques for chemical applications is missing from the scanning probe toolbox: chemical recognition of organic molecules. Here, we show that maps of the minima of frequency shift-distance curves extracted from 3D data cubes contain characteristic contrast. A detailed theoretical analysis based on density functional theory and molecular mechanics shows that these features are characteristic for the investigated species. Structurally similar but chemically distinct molecules yield significantly different features. We find that the van der Waals and Pauli interaction, together with the specific adsorption geometry of a given molecule on the surface, accounts for the observed contrast.

Mapping the electrostatic force field of single molecules from high-resolution scanning probe images.

How electronic charge is distributed over a molecule determines to a large extent its chemical properties. Here, we demonstrate how the electrostatic force field, originating from the inhomogeneous charge distribution in a molecule, can be measured with submolecular resolution. We exploit the fact that distortions typically observed in high-resolution atomic force microscopy images are for a significant part caused by the electrostatic force acting between charges of the tip and the molecule of interest. By finding a geometrical transformation between two high-resolution AFM images acquired with two different tips, the electrostatic force field or potential over individual molecules and self-assemblies thereof can be reconstructed with submolecular resolution.

Submolecular Resolution Imaging of Molecules by Atomic Force Microscopy: The Influence of the Electrostatic Force.

The forces governing the contrast in submolecular resolution imaging of molecules with atomic force microscopy (AFM) have recently become a topic of intense debate. Here, we show that the electrostatic force is essential to understand the contrast in atomically resolved AFM images of polar molecules. Specifically, we image strongly polarized molecules with negatively and positively charged tips. A contrast inversion is observed above the polar groups. By taking into account the electrostatic forces between tip and molecule, the observed contrast differences can be reproduced using a molecular mechanics model. In addition, we analyze the height dependence of the various force components contributing to the high-resolution AFM contrast.

Intermolecular contrast in atomic force microscopy images without intermolecular bonds.

Intermolecular features in atomic force microscopy images of organic molecules have been ascribed to intermolecular bonds. A recent theoretical study [P. Hapala et al., Phys. Rev. B 90, 085421 (2014)] showed that these features can also be explained by the flexibility of molecule-terminated tips. We probe this effect by carrying out atomic force microscopy experiments on a model system that contains regions where intermolecular bonds should and should not exist between close-by molecules. Intermolecular features are observed in both regions, demonstrating that intermolecular contrast cannot be directly interpreted as intermolecular bonds.

Removal of GPI-anchored membrane proteins causes clustering of lipid microdomains in the apical head area of porcine sperm.

The release of extracellular proteins is a part of the sperm capacitation process; this allows the sperm surface reorganization that enables the sperm to fertilize an oocyte. Some of the components released are 'decapacitation factors', an uncoordinated or early release of which may cause inappropriate surface destabilization and premature capacitation. We studied the involvement of glycosylphosphatidylinositol-anchored proteins (GPI-APs) in sperm capacitation, and reported that CD52 and CD55 exhibit bicarbonate-dependent release during in vitro sperm capacitation. Treating sperm with phosphatidylinositol-specific phospholipase C (PIPLC) resulted in the enzymatic cleavage of CD55, in both capacitating and noncapacitating conditions. Moreover, PIPLC treatment in noncapacitating conditions caused surface reorganization events that included exposure of the ganglioside GM1, aggregation of flotillin-1, and the swelling of the apical acrosome region; all of which have been reported to be associated with sperm capacitation. The acrosomal swelling was monitored using wet mount atomic force microscopy, a new imaging technique that allows nanometer-level sperm surface measurements in samples hydrated with physiological buffer rather than dried. Despite these surface changes, PIPLC treatment in identical incubation conditions did not stimulate hyperactive sperm motility or protein tyrosine phosphorylation (other hallmarks of sperm capacitation in vitro). In full capacitating conditions (i.e., the presence of bicarbonate and albumin), PIPLC treatment caused sperm deterioration. The possible role of GPI-APs removal from the sperm surface during sperm capacitation is discussed.

Suppression of electron-vibron coupling in graphene nanoribbons contacted via a single atom.

Graphene nanostructures, where quantum confinement opens an energy gap in the band structure, hold promise for future electronic devices. To realize the full potential of these materials, atomic-scale control over the contacts to graphene and the graphene nanostructure forming the active part of the device is required. The contacts should have a high transmission and yet not modify the electronic properties of the active region significantly to maintain the potentially exciting physics offered by the nanoscale honeycomb lattice. Here we show how contacting an atomically well-defined graphene nanoribbon to a metallic lead by a chemical bond via only one atom significantly influences the charge transport through the graphene nanoribbon but does not affect its electronic structure. Specifically, we find that creating well-defined contacts can suppress inelastic transport channels.

Quantitative atomic resolution force imaging on epitaxial graphene with reactive and nonreactive AFM probes.

Atomic force microscopy (AFM) images of graphene and graphite show contrast with atomic periodicity. However, the contrast patterns vary depending on the atomic termination of the AFM tip apex and the tip-sample distance, hampering the identification of the atomic positions. Here, we report quantitative AFM imaging of epitaxial graphene using inert (carbon-monoxide-terminated) and reactive (iridium-terminated) tips. The atomic image contrast is markedly different with these tip terminations. With a reactive tip, we observe an inversion from attractive to repulsive atomic contrast with decreasing tip-sample distance, while a nonreactive tip only yields repulsive atomic contrast. We are able to identify the atoms with both tips at any tip-sample distance. This is a prerequisite for future structural and chemical analysis of adatoms, defects, and the edges of graphene nanostructures, crucial for understanding nanoscale graphene devices.