165 research outputs found
Controlling the electronic properties of van der Waals heterostructures by applying electrostatic design
Van der Waals heterostructures based on the heteroassembly of 2D materials represent a recently developed class of materials with promising properties especially for optoelectronic applications. The alignment of electronic energy bands between consecutive layers of these heterostructures crucially determines their functionality. In the present paper, relying on dispersion-corrected density-functional theory calculations, we present electrostatic design as a promising tool for manipulating this band alignment. The latter is achieved by inserting a layer of aligned polar molecules between consecutive transition-metal dichalcogenide (TMD) sheets. As a consequence, collective electrostatic effects induce a shift of as much as 0.3âeV in the band edges of successive TMD layers. Building on that, the proposed approach can be used to design electronically more complex systems, like quantum cascades or quantum wells, or to change the type of band lineup between type II and type I
van der Waals Interaction Activated Strong Electronic Coupling at the Interface between Chloro Boron-Subphthalocyanine and Cu(111)
In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby âpullingâ the molecule toward the surface. This reveals two adsorption regimes with significantly different adsorption distances, molecular conformations, and adsorbate-induced changes of the work function. Notably, the above-mentioned massive hybridization of electronic states, also observed in photoelectron spectroscopy, is obtained solely for one of the regimes. We show that this regime is accessible only as a consequence of the planarization of the molecular backbone resulting from the van der Waals attraction between the molecule and the surface. The results of this study indicate that for certain metalâmolecule combinations unusually strong interfacial electronic interactions can be triggered by van der Waals forces creating a situation that differs from the usually described cases of physisorptive and chemisorptive interactions
The size of electron-hole pairs in pi conjugated systems
We have performed momentum dependent electron energy-loss studies of the
electronic excitations in sexithiophene and compared the results to those from
parent oligomers. Our experiment probes the dynamic structure factor
S(q,omega)and we show that the momentum dependent intensity variation of the
excitations observed can be used to extract the size of the electron-hole pair
created in the excitation process. The extension of the electron-hole pairs
along the molecules is comparable to the length of the molecules and thus maybe
only limited by structural constraints. Consequently, the primary
intramolecular electron-hole pairs are relatively weakly bound. We find no
evidence for the formation of excitations localized on single thiophene units.Comment: RevTex, 3 figures, to appear in Physical Review Letter
Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111)
In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to âCl-upâ molecules with the BâCl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules. In contrast to the standard interpretation of adsorption of nonplanar molecules in the phthalocyanine family, we find no evidence for âCl-downâ molecules where the BâCl moiety would be pointing toward the Cu surface. We show computationally that such a configuration is unstable and thus is highly unlikely to occur for ClB-SubPc on Cu(111). Using these assignments, we discuss the different self-assembly motifs in the submonolayer coverage regime. The combination of DFT and STM is essential to gain a full atomistic understanding of the surfaceâmolecule interactions, and our findings imply that phthalocyanines may undergo surface-catalyzed reactions hitherto not considered. Our results also indicate that care has to be taken when analyzing possible adsorption configurations of polar members of the phthalocyanine family, especially when they are adsorbed on comparably reactive surfaces like Cu(111)
Generalised massive gravity one-loop partition function and AdS/(L)CFT
The graviton 1-loop partition function is calculated for Euclidean
generalised massive gravity (GMG) using AdS heat kernel techniques. We find
that the results fit perfectly into the AdS/(L)CFT picture. Conformal
Chern-Simons gravity, a singular limit of GMG, leads to an additional
contribution in the 1-loop determinant from the conformal ghost. We show that
this contribution has a nice interpretation on the conformal field theory side
in terms of a semi-classical null vector at level two descending from a primary
with conformal weights (3/2,-1/2).Comment: 25 p., 2 jpg figs, v2: added 6 lines of clarifying text after Eq.
(2.38
Complex Stoichiometry-Dependent Reordering of 3,4,9,10-Perylenetetracarboxylic Dianhydride on Ag(111) upon K Intercalation
Alkali metal atoms are frequently used for simple yet efficient n-type doping of organic semiconductors and as an ingredient of the recently discovered polycyclic aromatic hydrocarbon superconductors. However, the incorporation of dopants from the gas phase into molecular crystal structures needs to be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. Here, we report that potassium intercalation into the pristine 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large KxPTCDA domains. The emerging structures are analyzed by low-temperature scanning tunneling microscopy, scanning tunneling hydrogen microscopy (ST[H]M), and low-energy electron diffraction as a function of the stoichiometry. The analysis of the measurements is corroborated by density functional theory calculations. These turn out to be essential for a correct interpretation of the experimental ST[H]M data. The epitaxy types for all intercalated stages are determined as point-on-line. The K atoms adsorb in the vicinity of the oxygen atoms of the PTCDA molecules, and their positions are determined with sub-\uc5ngstr\uf6m precision. This is a crucial prerequisite for the prospective assessment of the electronic properties of such composite films, as they depend rather sensitively on the mutual alignment between donor atoms and acceptor molecules. Our results demonstrate that only the combination of experimental and theoretical approaches allows for an unambiguous explanation of the pronounced reordering of KxPTCDA/Ag(111) upon changing the K content
Large scale numerical investigation of excited states in poly(phenylene)
A density matrix renormalisation group scheme is developed, allowing for the
first time essentially exact numerical solutions for the important excited
states of a realistic semi-empirical model for oligo-phenylenes. By monitoring
the evolution of the energies with chain length and comparing them to the
experimental absorption peaks of oligomers and thin films, we assign the four
characteristic absorption peaks of phenyl-based polymers. We also determine the
position and nature of the nonlinear optical states in this model.Comment: RevTeX, 10 pages, 4 eps figures included using eps
Holographic two-point functions for 4d log-gravity
We compute holographic one- and two-point functions of critical
higher-curvature gravity in four dimensions. The two most important operators
are the stress tensor and its logarithmic partner, sourced by ordinary massless
and by logarithmic non-normalisable gravitons, respectively. In addition, the
logarithmic gravitons source two ordinary operators, one with spin-one and one
with spin-zero. The one-point function of the stress tensor vanishes for all
Einstein solutions, but has a non-zero contribution from logarithmic gravitons.
The two-point functions of all operators match the expectations from a
three-dimensional logarithmic conformal field theory.Comment: 35 pages; v2: typos corrected, added reference; v3: shorter
introduction, minor changes in the text in section 3, added reference;
published versio
Short-cut to new anomalies in gravity duals to logarithmic conformal field theories
Various massive gravity theories in three dimensions are conjecturally dual
to logarithmic conformal field theories (LCFTs). We summarise the status of
these conjectures. LCFTs are characterised by the values of the central charges
and the so-called "new anomalies". We employ a short-cut to calculate these new
anomalies in generalised massive gravity and in the recently proposed
higher-derivative gravity theories with holographic c-theorem. Both cases
permit LCFTs exhibiting intriguing features, like rank three Jordan cells or
non-zero central charges. Finally, as an example we discuss in some detail the
partially massless version of new massive gravity, a theory with several
special properties that we call "partially massless gravity".Comment: 34 pages, 2 figures; v2: added references; v3: Several rewordings in
the introduction and section 2, added references. Matches published versio
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