ABSTRACT
Sb is a three-dimensional Peierls insulator. The Peierls instability gives rise to doubling of the translational period along the [111] direction and alternating van der Waals and covalent bonding between (111) atomic planes. At the (111) surface of Sb, the Peierls condition is violated, which in theory can give rise to properties differing from the bulk. The atomic and electronic structure of the (111) surface of Sb have been simulated by density functional theory calculations. We have considered the two possible (111) surfaces, containing van der Waals dangling bonds or containing covalent dangling bonds. In the models, the surfaces are infinite and the structure is defect free. Structural optimization of the model containing covalent dangling bonds results in strong deformation, which is well described by a topological soliton within the Su-Schrieffer-Heeger model centered about 25 Å below the surface. The electronic states associated with the soliton see an increase in the density of states (DOS) at the Fermi level by around an order of magnitude at the soliton center. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements reveal two distinct surface regions, indicating that there are different surface regions cleaving van der Waals and covalent bonds. The DFT is in good agreement with the STM/STS experiments.
ABSTRACT
Gaining control over the spatial distribution of nanoparticles in composite polymer materials is a relevant goal for a range of nanotechnology applications. Promising methods to produce nanoparticles directly in the polymer matrix rely on their self-assembly from the atoms that are generated due to the photodestruction of the precursor additive. Such materials are known as photoinduced nanocomposites. In this work, we theoretically study the possibility of producing tailored nanoparticle distributions in such materials by the local modification of their physical properties. For instance, laser irradiation may cause a permanent free-volume expansion (laser swelling), which results in a substantial change in the diffusivity of the aggregating atoms. The modeling shows that the nanoparticles tend to accumulate in the domains where the diffusivity is greater. Additionally, the variation of the matrix properties may result in spatial modulation of the atom-matrix interaction energy and thus modulation of the atom solubility in the matrix. This phenomenon can also affect the NP spatial distribution. This paper formulates the problem of the precipitation phase transition from the supersaturated solution in a polymer solvent which is "frozen" in the spatially nonuniform state. The basic traits of this phenomenon are studied by means of an analytical model. Then the NP growth is simulated using a lattice model.
ABSTRACT
We study the thermo-physical and photoluminescence (PL) properties of cadmium-(bis)dodecylthiolate (Cd(C12H25S)2). Significant attention is drawn to characterization of Cd(C12H25S)2 by different methods. The laser-induced PLs of Cd(C12H25S)2 and Cd(C12H25S)2/(polymethyl methacrylate) (PMMA) composites are studied. Samples of Cd(C12H25S)2/PMMA are synthesized by the polymerization method. Ultraviolet (UV)-pulsed laser irradiation of the samples under relatively small fluences leads to the formation of induced PL with the maximum near the wavelength of 600 nm. This process can be attributed to the transformation of Cd(C12H25S)2 within the precursor grains. Another PL peak at 450-500 nm, which appears under the higher fluences, relies on the formation of CdS complexes with a significant impact of the polymer matrix.
