Annealing mechanisms of point defects in silicon carbide : a theoretical investigation

Eva RaulsPaderborn, Univ., Diss., 200

Phosphorus donors in highly strained silicon

The hyperfine interaction of phosphorus donors in fully strained Si thin films grown on virtual Si$_{1-x}$Ge$_x$ substrates with $x\leq 0.3$ is determined via electrically detected magnetic resonance. For highly strained epilayers, hyperfine interactions as low as 0.8 mT are observed, significantly below the limit predicted by valley repopulation. Within a Green's function approach, density functional theory (DFT) shows that the additional reduction is caused by the volume increase of the unit cell and a local relaxation of the Si ligands of the P donor.Comment: 12 pages, 3 figure

Towards dielectric relaxation at a single molecule scale

Dielectric relaxation lies at the heart of well-established techniques of dielectric spectroscopy essential to diverse fields of research and technology. We report an experimental route for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of a single molecule. We use the method of radio frequency scanning tunneling microscopy to excite a single molecule junction based on a polar substituted helicene molecule by an electric field oscillating at 2–5 GHz. We detect the dielectric relaxation of the single molecule junction indirectly via its effect of power dissipation, which causes lateral displacement. From our data we determine a corresponding relaxation time of about 300 ps—consistent with literature values of similar helicene derivatives obtained by conventional methods of dielectric spectroscopy.publishedVersio

Consistent Atomic Geometries and Electronic Structure of Five Phases of Potassium Niobate from Density-Functional Theory

We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization of KNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material

Preserving Charge and Oxidation State of Au(III) Ions in an Agent-Functionalized Nanocrystal Model System

Supporting functional molecules on crystal facets is an established technique in nanotechnology. To preserve the original activity of ionic metallorganic agents on a supporting template, conservation of the charge and oxidation state of, the active center is indispensable. We. present a model system of a metallorganic agent that, indeed, fulfills this design criterion on a technologically relevant metal support With potential Impact on Au(III)-porphyrin-functionalized nanoparticles for an improved anticancer-drug delivery. Employing scanning tunneling microscopy and -spectroscopy in combination with photoemission spectroscopy,we clarify at the single-molecule level the underlying mechanisms of this exceptional adsorption mode. It is based on the balance between a high-energy oxidation state and an electrostatic screening-response of the surface (image charge). Modeling with first principles methods reveals submolecular details of the metal-ligand bonding interaction and completes the study by providing an Illustrative electrostatic.. model relevant for ionic metalorganic agent molecules, in general

Computational Study of the Optimal P[n]A structure for CO2 Adsorption

Carbon dioxide (CO2) is the main greenhouse gas that contributes to the global warming. Therefore, CO2 adsorption is very urgent in the fight to limit global warming below 1.5 degrees Celsius. In this report, the interaction between CO2 with different structures of pillar[n]arene (P[n]A) is studied by using DFTB and DFT calculations, in order to understand the effect of P[n]A (with n=4, 5, and 6) cavity sizes on CO2 adsorption. The P[n]A structures physisorb CO2 at three principally different positions called cavity-in, top-in, and top-out. The adsorbed CO2-cavity-in at P[4]A has the highest binding energy. The adsorbed CO2 at the other positions has similar binding energies on P[4]A, P[5]A, and P[6]A, because hydrogen bonding plays a major role for the interaction at the hydroxyl group. The number of CO2 molecules that can be adsorbed at the cavity site depends on the cavity size of P[n]A. The bigger the cavity site, the larger the number of CO2 molecules that can be adsorbed before saturation is achieved. We also observed that the adsorbed CO2 molecules can interact with each other, leading to an increase of the binding energy and highlighting the promising CO2 capture capabilities of P[n]A structures.publishedVersio

Extended One-Dimensional Supramolecular Assembly on a Stepped Surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag(110) surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility. ©2008 The American Physical Societ

Structural variety of 5-fluoroarene-2-aminopyrimidine in comparison to 2-aminopyrimidine silver(I) coordination polymers: progress report and overview

Eberhard J, Stoll I, Brockhinke R, et al. Structural variety of 5-fluoroarene-2-aminopyrimidine in comparison to 2-aminopyrimidine silver(I) coordination polymers: progress report and overview. Crystengcomm. 2013;15(21):4225-4248.A variety of fluoroarene-2-aminopyrimidine (FAP) silver(I) coordination polymers (CPs) has been synthesized based on newly synthesized FAP derivatives, namely 5-(p-methoxytetrafluorophenyl)-2-aminopyrimidine (OFAP) and 5-(p-dimethylaminotetrafluorophenyl)-2-aminopyrimidine (NFAP), and different counterions (OTf-, TFA(-), ClO4-, NO3-). Their solid-state assembly as well as optical properties in terms of luminescence and infrared (IR) spectroscopy were investigated. Out of the several structures described herein, we obtained isomorphic CPs to previous studies (5, 9), a CP architecture with a short Ag-Ag distance (8, 3.049 angstrom), but also polymorphic crystals of [Ag(nfap)NO3](n) (11a, 11b) and the latter showed differences in color and luminescence emission. Polymorphism gives an unparalleled possibility to investigate the origin of such phenomena since luminescence emission is quite often observed for these silver-hybrid solid-state materials and in several cases Ag center dot center dot center dot Ag interactions are attributed for this phenomenon. We show that this explanation does not necessarily have to be the only one. Therefore we focus also on structural relationships and differences in a comprehensive comparison of our own and other known systems to start a more systematic description of the rich coordination capabilities of FAP and congeneric 2-aminopyrimidine (2-AP) based silver(I) coordination polymers and networks. Density functional theory (DFT) calculations with periodic boundary conditions based on a plane wave basis are used to better understand the electronic structure of these crystalline materials. To complete the picture, steady-state spectroscopy studies (UV-Vis, fluorescence, IR) on all ligands and 2-AP itself were conducted as well as re-examination of the first reported CP of 2-AP and Ag-I under the above aspects