73 research outputs found
RoomâTemperature Transport Properties of Graphene with Defects Derived from OxoâGraphene
In recent years, graphene oxide has been considered as a soluble precursor of graphene for electronic applications. However, the performance lags behind that of graphene due to lattice defects. Here, the relation between the density of defects in the range of 0.2â% and 1.5â% and the transport properties is quantitatively studied. Therefore, the related flakes of monolayers of graphene were prepared from oxoâfunctionalized graphene (oxoâG). The morphologic structure of oxoâG was imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Fieldâeffect mobility values were determined to range between 0.3â
cm2âVâ1âsâ1 and 33.2â
cm2âVâ1âsâ1, which were inversely proportional to the density of defects. These results provide the first quantitative description of the density of defects and transport properties, which plays an important role for potential applications
Molecule signatures in photoluminescence spectra of transition metal dichalcogenides
Monolayer transition metal dichalcogenides (TMDs) show an optimal
surface-to-volume ratio and are thus promising candidates for novel molecule
sensor devices. It was recently predicted that a certain class of molecules
exhibiting a large dipole moment can be detected through the activation of
optically inaccessible (dark) excitonic states in absorption spectra of
tungsten-based TMDs. In this work, we investigate the molecule signatures in
photoluminescence spectra in dependence of a number of different experimentally
accessible quantities, such as excitation density, temperature as well as
molecular characteristics including the dipole moment and its orientation,
molecule-TMD distance, molecular coverage and distribution. We show that under
certain optimal conditions, even room temperature detection of molecules can be
achieved
Evidence for electron transfer between graphene and nonâcovalently bound Ïâsystems
Hybridizing graphene and molecules possess a high potential for developing materials for new applications. However, new methods to characterize such hybrids must be developed. Herein, the wetâchemical nonâcovalent functionalization of graphene with cationic Ïâsystems is presented and the interaction between graphene and the molecules is characterized in detail. A series of tricationic benzimidazolium salts with various steric demand and counterions was synthesized, characterized and used for the fabrication of graphene hybrids. Subsequently, the doping effects were studied. The molecules are adsorbed onto graphene and studied by Raman spectroscopy, XPS as well as ToFâSIMS. The charged Ïâsystems show a pâdoping effect on the underlying graphene. Consequently, the tricationic molecules are reduced through a partial electron transfer process from graphene, a process which is accompanied by the loss of counterions. DFT calculations support this hypothesis and the strong pâdoping could be confirmed in fabricated monolayer graphene/hybrid FET devices. The results are the basis to develop sensor applications, which are based on analyte/molecule interactions and effects on doping
Identification of the Irreversible Redox Behavior of Highly Fluorescent Benzothiadiazoles
Redox switches are applied in various fields of research, including molecular lifts, electronic devices and sensors. Switching the absorbance between UV and Vis/NIR by redox processes is of interest for applications in light harvesting or biomedicine. Here, we present a series of pushâpull benzothiadiazole derivatives with high fluorescence quantum yields in solution and in the crystalline solid state. Spectroelectrochemical analysis reveals the switching of UVâabsorption in the neutral state to Vis/NIR absorption in the reduced state. We identify the partial irreversibility of the switching process, which appears to be reversible on the cyclic voltammetry timescale
Resolution of intramolecular dipoles and push-back effect of individual molecules on a metal surface
Molecules consisting of a donor and an acceptor moiety can exhibit large
intrinsic dipole moments. Upon deposition on a metal surface, the dipole may be
effectively screened and the charge distribution altered due to hybridization
with substrate electronic states. Here, we deposit Ethyl-Diaminodicyanoquinone
molecules, which exhibit a large dipole moment in gas phase, on a Au(111)
surface. Employing a combination of scanning tunneling microscopy and
non-contact atomic force microscopy, we find that a significant dipole moment
persists in the flat-lying molecules. Density-functional theory calculations
reveal that the dipole moment is even increased on the metal substrate as
compared to the gas phase. We also show that the local contact potential across
the molecular islands is decreased by several tens of meV with respect to the
bare metal. We explain this by the induced charge-density redistribution due to
the adsorbed molecules, which confine the substrate's wavefunction at the
interface. Our local measurements provide direct evidence of this so-called
push-back or cushion effect at the scale of individual molecules.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Journal of Physical
Chemistry
Variations of vibronic states in densely-packed structures of molecules with intramolecular dipoles
Electrostatic potentials strongly affect molecular energy levels and charge
states, providing the fascinating opportunity of molecular gating. Their
influence on molecular vibrations remains less explored. Here, we investigate
Ethyl-Diaminodicyanoquinone molecules on a monolayer of MoS on Au(111)
using scanning tunneling and atomic force microscopy and spectroscopy. These
molecules exhibit a large dipole moment in gas phase, which we find to
(partially) persist on the MoS monolayer. The self-assembled structures
consist of chains, where the dipoles of neighboring molecules are aligned
anti-parallel. Thanks to the decoupling efficiency of the molecular states from
the metal by the MoS interlayer, we resolve vibronic states of the
molecules, which vary in intensity depending on the molecular surrounding. We
suggest that the vibrations are strongly damped by electrostatic interactions
with the environment
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