193 research outputs found
Electronic Excitations from a Perturbative LDA+GdW Approach
We discuss an efficient approach to excited electronic states within
ab-initio many-body perturbation theory (MBPT). Quasiparticle corrections to
density-functional theory result from the difference between metallic and
non-metallic dielectric screening. They are evaluated as a small perturbation
to the DFT-LDA band structure, rather than fully calculating the self energy
and evaluating its difference from the exchange-correlation potential. The
dielectric screening is desribed by a model, which applies to bulk crystals, as
well as, to systems of reduced dimension, like molecules, surfaces, interfaces,
and more. The approach also describes electron-hole interaction. The resulting
electronic and optical spectra are slightly less accurate but much faster to
calculate than a full MBPT calculation. We discuss results for bulk silicon and
argon, for the Si(111)-(2x1) surface, the SiH4 molecule, an argon-aluminum
interface, and liquid argon
Redshift of excitons in carbon nanotubes caused by the environment polarizability
Optical excitations of molecular systems can be modified by their physical
environment. We analyze the underlying mechanisms within many-body perturbation
theory, which is particularly suited to study non-local polarizability effects
on the electronic structure. Here we focus on the example of a semiconducting
carbon nanotube, which observes redshifts of its excitons when the tube is
touched by another nanotube or other physisorbates. We show that the redshifts
mostly result from the polarizability of the attached ad-system. Electronic
coupling may enhance the redshifts, but depends very sensitively on the
structural details of the contact
Scanning Quantum Dot Microscopy
Interactions between atomic and molecular objects are to a large extent
defined by the nanoscale electrostatic potentials which these objects produce.
We introduce a scanning probe technique that enables three-dimensional imaging
of local electrostatic potential fields with sub-nanometer resolution.
Registering single electron charging events of a molecular quantum dot attached
to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we
quantitatively measure the quadrupole field of a single molecule and the dipole
field of a single metal adatom, both adsorbed on a clean metal surface. Because
of its high sensitivity, the technique can record electrostatic potentials at
large distances from their sources, which above all will help to image complex
samples with increased surface roughness.Comment: main text: 5 pages, 4 figures, supplementary information file: 4
pages, 2 figure
Immunohistochemical localization patterns for vimentin and other intermediate filaments in calcified ovarian fibrothecoma
PROBLEM: To describe immunohistochemical features encountered in ovarian fibrothecoma with correlation to clinical presentation and surgical management. METHOD OF STUDY: A female age 75 presented for evaluation of melena. The patient reported total abdominal hysterectomy and removal of both ovaries 40 years earlier. RESULTS: CA-125 was normal and there was no evidence of hyperestrogen effect. Pelvic CT revealed a partially calcified 7 cm pelvic mass without adenopathy or ascites; ultrasound was confirmatory. Endoscopy identified three benign intestinal tubular adenomas. Following laparoscopic excision of the pelvic tumor immunohistochemichal analysis of the mass showed negative staining for keratin, S100 protein, inhibin, calretinin, melan A, smooth muscle actin, CD34, CD117, and desmin. The tissue was positive for vimentin, however. CONCLUSION: Ovarian fibrothecomas represent an ovarian stromal neoplasm developing in a wide spectrum of clinical settings. Particularly if oophorectomy is stated to have been performed remote from the time of index presentation, the status of the ovaries must be considered whenever pelvic pathology is encountered. We describe a calcified ovarian fibrothecoma identified during gastroenterology investigation and confirmed immunohistochemically via high amplitude vimentin signal
Cooperative mechanism for anchoring highly polar molecules at an ionic surface
Schütte J, Bechstein R, Rohlfing M, Reichling M, Kühnle A. Cooperative mechanism for anchoring highly polar molecules at an ionic surface. Physical Review B. 2009;80(20):205421.Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed stable structures. Density-functional theory calculations yield a diffusion barrier of about 0.5 eV for individual molecules suggesting that they are mobile at room temperature. Furthermore, it is predicted that the molecules can form trimers in a configuration allowing all molecules to attain their optimum adsorption position on the substrate. As the trimer geometry facilitates hydrogen bonding between the molecules within the trimer, we conclude that the stabilization of individual diffusing molecules into stable trimers is due to a cooperative mechanism involving polar interactions between molecules and substrate as well as hydrogen bonding between molecules
Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2
The research field of two dimensional (2D) materials strongly relies on
optical microscopy characterization tools to identify atomically thin materials
and to determine their number of layers. Moreover, optical microscopy-based
techniques opened the door to study the optical properties of these
nanomaterials. We presented a comprehensive study of the differential
reflectance spectra of 2D semiconducting transition metal dichalcogenides
(TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up
to six layers. We analyzed the thickness-dependent energy of the different
excitonic features, indicating the change in the band structure of the
different TMDC materials with the number of layers. Our work provided a route
to employ differential reflectance spectroscopy for determining the number of
layers of MoS2, MoSe2, WS2, and WSe2.Comment: Main text (3 Figures) and Supp. Info. (23 Figures
Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110)
Greuling A, Rahe P, Kaczmarski M, Kühnle A, Rohlfing M. Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110). Journal of Physics : Condensed Matter. 2010;22(34): 345008.The adsorption behavior of trimesic acid (TMA) on rutile TiO2(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functional theory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imaging reveals individual molecules, centered above the surface titanium rows. Based on the NC-AFM results alone it is difficult to deduce whether the molecules are lying flat or standing upright on the surface. To elucidate the detailed adsorption geometry, we perform DFT calculations, considering a large number of different adsorption positions. Our DFT calculations suggest that single TMA molecules adsorb with the benzene ring parallel to the surface plane. In this configuration, two carboxylic groups can anchor to the surface in a bidentate fashion with the oxygen atoms binding to surface titanium atoms while the hydrogen atoms approach oxygen atoms within the bridging oxygen rows. The most favorable adsorption position is obtained in the presence of a hydroxyl defect, allowing for additional binding of the third carboxylic group
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