Sven Stafström

Negative Poisson's ratios as a common feature of cubic metals

Poissons ratio is, for specified directions, the ratio of a lateral contraction to the longitudinal extension during the stretching of a material. Although a negative Poissons ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, this property is generally believed to be rare in crystalline solids. In contrast to this belief, 69% of the cubic elemental metals have a negative Poissons ratio when stretched along the [110] direction. For these metals, we find that correlations exist between the work function and the extremal values of Poissons ratio for this stretch direction, which we explain using a simple electron-gas model. Moreover, these negative Poissons ratios permit the existence, in the orthogonal lateral direction, of positive Poissons ratios up to the stability limit of 2 for cubic crystals. Such metals having negative Poissons ratios may find application as electrodes that amplify the response of piezoelectric sensors.

The chemical and electronic structure of the interface between aluminum and polythiophene semiconductors

We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core‐level x‐ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl‐substituted poly‐3‐octylthiophene and (ii) the α‐sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythio...

Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states

The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq3) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq3 molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq3 molecule.

Optical absorption of poly(3-alkylthiophenes) at low temperatures

Abstract We present ultra-violet visible optical absorption spectra of thin solid films of four poly(3-alkylthiophenes) between 10 K and room temperature. The thermochromic shift in the optical absorption turns out to be detectable down to about 200 K. An interpretation of the fine structure in the optical spectra is also made in terms of vibronic structure caused by the CC stretching vibration of the thiophene ring close to 1460 cm −1 .

Mechanisms of halogen-based covalent self-assembly on metal surfaces

We computationally study the reaction mechanisms of halogen-based covalent self-assembly, a major route for synthesizing molecular nanostructures and nanographenes on surfaces. Focusing on biphenyl as a small model system, we describe the dehalogenation, recombination, and diffusion processes. The kinetics of the different processes are also investigated, in particular how diffusion and coupling barriers affect recombination rates. Trends across the periodic table are derived from three commonly used close-packed (111) surfaces (Cu, Ag, and Au) and two halogens (Br and I). We show that the halogen atoms can poison the surface, thus hindering long-range ordering of the self-assembled structures. Finally, we present core-level shifts of the relevant carbon and halogen atoms, to provide reference data for reliably detecting self-assembly without the need for atomic-resolution scanning tunneling microscopy.

Electron localization and the transition from adiabatic to nonadiabatic charge transport in organic conductors

This tutorial review describes an atomistic simulation approach to studies of dynamics of charge transport in pi-conjugated molecular or polymer based materials. The approach, which is termed electron-lattice dynamics, is based on the Ehrenfest theorem and includes nonadiabatic transport processes. The equations of motion for both the electrons and the nuclei are solved simultaneously using a Runge-Kutta method. We show that for ideal systems without disorder and thermal fluctuations in the electronic coupling between the constituent elements that the electron transport can be described as an adiabatic polaron drift process. In the presence of thermal fluctuations caused by, e.g., phenyl ring torsions along a poly(paraphenylene vinylene) chain, there are extensive variations in the electronic coupling along the polymer chain which results in nonadiabatic hopping transport.

Tuning the Supramolecular Chirality of One- and Two-Dimensional Aggregates with the Number of Stereogenic Centers in the Component Porphyrins

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.

Predicted stability of a new aza[60]fullerene molecule, C48N12

Optimized geometries and total energies of a series of aza[60]fullerenes, C60−2nN2n, have been calculated by density functional theory using B3LYP/6-31G*. The minimum total energy of C58N2 is the isomer with a pair of N atoms sitting in the (1,4) positions of the [60]fullerene. Maximum relative stability in the series is obtained for the isomer of C48N12 with one N per pentagon and S6 point group symmetry. The nitrogen content in C48N12 is in close agreement with the observed nitrogen concentration in fullerene-like onions of CNx. C48N12 is iso-electronic with C-60-12 and has an ionization potential which is 1.6 eV lower than that of C60.

Reactivity of curved and planar carbon–nitride structures

The reactivity of different carbon–nitride structures has been studied using density functional theory calculations. The studies involve C59N and clusters of curved and planar CNx structures. Nitrogen is shown to lower the energy of pentagon defects in the graphite like structures, whereas heptagons are unlikely to be present. From this observation, it follows that nitrogen stimulates growth of fullerene like structures in CNx. The presence of nitrogen also increases the reactivity of the carbon atoms around the nitrogen. This leads to cross linking between basal planes which can explain the hardness and elasticity of CNx films.

Zipping Up: Cooperativity Drives the Synthesis of Graphene Nanoribbons

We investigate the cooperative effects controlling the synthesis of a graphene nanoribbon on the Au(111) surface starting from an anthracene polymer using density functional calculations including van der Waals interactions. We focus on the high-temperature cyclodehydrogenation step of the reaction and find that the reaction proceeds by simultaneously transferring two H-atoms from the anthracene units to the Au surface, leaving behind a C-C bond in the process. This step is significantly more favorable than the three other potential reaction paths. Moreover, we find that successive dehydrogenations proceed from one end of the polyanthracene and propagate step-by-step through the polymer in a domino-like fashion.