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1.
Proc Natl Acad Sci U S A ; 112(45): E6101-10, 2015 Nov 10.
Artigo em Inglês | MEDLINE | ID: mdl-26512115

RESUMO

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorph-dependent dispersion-induced substrate-molecule interactions (e.g., -100 kcal mol(-1) to -150 kcal mol(-1) for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70-110 kcal mol(-1)) and entropy effects (25-40 kcal mol(-1) at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations.

2.
Chemphyschem ; 15(16): 3484-8, 2014 Nov 10.
Artigo em Inglês | MEDLINE | ID: mdl-25146424

RESUMO

Molecules of (5,10,15,20-tetraundecylporphyrinato)-copper(II) [(TUP)Cu] can self-assemble into four different polymorphs at the interface between highly oriented pyrolytic graphite and 1-octanoic acid. Scanning tunneling microscopy (STM) reveals that it is possible to combine the global control over monolayer structure, provided by the composition and concentration of the supernatant solution, with local control, from nanomanipulation by the STM tip. In the initially formed monolayer, with a polymorph composition governed by the concentration of (TUP)Cu in the supernatant solution, the exchange of molecules physisorbed at the solid/liquid interface with those in the liquid is very limited. By using a nanoshaving procedure at the tip, defects are created in the monolayer, and these serve as local manipulation sites to create domains of higher or lower molecular density, and to incorporate a second molecular species, (TUP)Co into the monolayer of (TUP)Cu.


Assuntos
Nanoestruturas/química , Porfirinas/química , Caprilatos/química , Grafite/química , Microscopia de Tunelamento , Nanoestruturas/ultraestrutura
3.
Phys Chem Chem Phys ; 15(30): 12451-8, 2013 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-23620134

RESUMO

Self-assembled monolayers of meso-5,10,15,20-tetrakis(undecyl)porphyrin copper(II) on a graphite/1-octanoic acid interface have been studied by Scanning Tunnelling Microscopy. Four distinct polymorphs were observed, varying in their unit cell size. Arrays of unit cells of the various polymorphs seamlessly connect to each other via shared unit cell vectors. The monolayers are not commensurate, but coincident with the underlying graphite substrate. The seamless transition between the polymorphs is proposed to be the result of an adaptation of the molecular conformations in the polymorphs and at the boundaries, which is enabled by the conformational freedom of the alkyl tails of these molecules.


Assuntos
Porfirinas/química , Adsorção , Caprilatos/química , Cobre/química , Grafite/química , Microscopia de Tunelamento , Conformação Molecular
4.
Langmuir ; 29(4): 1206-10, 2013 Jan 29.
Artigo em Inglês | MEDLINE | ID: mdl-23244532

RESUMO

Using conducting probe atomic force microscopy (CAFM), we show that electrical conductivity in oligothiophene molecular films deposited on SiO(2)/Si wafers is extremely sensitive to degree of crystalline order in the film. By locally distorting the molecular order in the films through the controlled application of pressure with the AFM tip, the lateral charge transport was reduced by factors varying from 2 to 10, even when no changes in the height of the film could be observed.

5.
J Am Chem Soc ; 134(16): 7186-92, 2012 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-22475214

RESUMO

A bis-Zn(salphen) structure shows extremely strong self-assembly both in solution as well as at the solid-liquid interface as evidenced by scanning tunneling microscopy, competitive UV-vis and fluorescence titrations, dynamic light scattering, and transmission electron microscopy. Density functional theory analysis on the Zn(2) complex rationalizes the very high stability of the self-assembled structures provoked by unusual oligomeric (Zn-O)(n) coordination motifs within the assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn(2)O(2) central unit. The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers with potential for new opto-electronic materials.

6.
Nano Lett ; 11(10): 4107-12, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21848283

RESUMO

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.

7.
Chem Commun (Camb) ; 47(34): 9666-8, 2011 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-21799980

RESUMO

The transition from low to high density 2D surface structures of copper porphyrins at a liquid/solid interface requires specific defects at which nearly all exchange of physisorbed molecules with those dissolved in the supernatant occurs.

8.
Nat Chem ; 2(9): 730-4, 2010 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-20729891

RESUMO

Atomic steps at the surface of a catalyst play an important role in heterogeneous catalysis, for example as special sites with increased catalytic activity. Exposure to reactants can cause entirely new structures to form at the catalyst surface, and these may dramatically influence the reaction by 'poisoning' it or by acting as the catalytically active phase. For example, thin metal oxide films have been identified as highly active structures that form spontaneously on metal surfaces during the catalytic oxidation of carbon monoxide. Here, we present operando X-ray diffraction experiments on a palladium surface during this reaction. They reveal that a high density of steps strongly alters the stability of the thin, catalytically active palladium oxide film. We show that stabilization of the metal, caused by the steps and consequent destabilization of the oxide, is at the heart of the well-known reaction rate oscillations exhibited during CO oxidation at atmospheric pressure.


Assuntos
Monóxido de Carbono/química , Paládio/química , Catálise , Cinética , Análise Numérica Assistida por Computador , Propriedades de Superfície , Difração de Raios X
9.
J Chem Phys ; 130(11): 114705, 2009 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-19317553

RESUMO

The correlation between molecular conductivity and mechanical properties (molecular deformation and frictional responses) of hexadecylsilane self-assembled monolayers was studied with conductive probe atomic force microscopy/friction force microscopy in ultrahigh vacuum. Current and friction were measured as a function of applied pressure, simultaneously, while imaging the topography of self-assembled monolayer molecule islands and silicon surfaces covered with a thin oxide layer. Friction images reveal lower friction over the molecules forming islands than over the bare silicon surface, indicating the lubricating functionality of alkylsilane molecules. By measuring the tunneling current change due to changing of the height of the molecular islands by tilting the molecules under pressure from the tip, we obtained an effective conductance decay constant (beta) of 0.52/A.

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