Electronic properties of molecular and low-dimensional materials
Recent Results
Evidence for a New Two-Dimensional C4H-Type Polymer Based on Hydrogenated Graphene
We have demonstrated that a new and stable C4H phase of hydrogenated graphene exists, which provides an upper limit to
storage of H on graphene under the applied conditions. We also report electronic structure calculations of C4H indicating that it is a wide bandgap semiconductor making it an attractive material for optoelectronics in the UV range. Moreover, the aromatic domains that are encircled by sp3 units may serve as a nanotemplate in a similar manner for smaller molecules comparable to the concave areas in boron nitride nanomesh.
Adv. Mater. 23, 4497 (2011)
Hole Transparent and Hole Blocking Transport in Single-Crystal-Like
Organic Heterojunction: When Rods Hold up Disks
Single-crystal-like organic heterojuntions are fabricated with disk-like molecules and different rodlike molecules.
Hole transparent and blocking transport are demonstrated with photoemission spectroscopy and field-effect
transistors. These results demonstrate a route to utilize adjustable interfacial electronic structure and control transport
behavior in developing functional organic crystalline devices and crystalline nanocircuits.
ACS Appl. Mater. Interfaces 3, 2195 (2011)
Interfacial energy level bending in a crystalline p/p-type organic heterostructure
A conduction channel was observed at the heterointerface of the crystalline p-type organic films
copper phthalocyanine (CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). Energy level bending
at the interface is confirmed by photoemission spectroscopy, which verifies a charge transfer
between CuPc and BP2T. This provides a further route to utilize interfacial electronic properties in
functional devices and also documents the importance of reconsidering the interfacial electronic
structure of organic heterostructures.
Appl. Phys. Lett. 98, 203303 (2011)
Identification of the electronic states of manganese phthalocyanine close to the Fermi level
The electronic properties of two metal phthalocyanines, CuPc and MnPc, have been investigated by a
combined experimental and theoretical approach. We demonstrate that the highest occupied molecular
orbital of MnPc has significant metal 3d character, which is in contrast to CuPc where this state is of pure
ligand p nature. This difference also explains the substantial variation in the ionization potential on going
from CuPc to MnPc. Moreover, our results allow to identify the 1eg symmetry of the Mn 3d state near the
Fermi level, and they provide insight into the magnetic anisotropy of the two phthalocyanines.
Chem. Phys. Lett. 505, 122 (2010)
Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene
We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches ∼1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp2 to sp3 in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials.
Nano Lett. 10, 3360 (2010)
Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic Devices
The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices. The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist time the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies.
Nano Lett. 10, 992 (2010)
Spin and Orbital Ground State of Co in Cobalt Phthalocyanine
The 3d orbital ground state of transition-metal ions that are incorporated in a molecular matrix determines
the total spin of the transition-metal ion as well as the spin anisotropy and thus the essential magnetic properties
of the corresponding molecule. However, there is little known to date on the exact 3d ground state of many
molecular systems, including quite complex molecular magnets as well as relatively simple systems such as,
for instance, cobalt phthalocyanine (CoPc). For the latter, there are contradictory theoretical predictions with
respect to the occupation of the various Co 3d electronic levels. We demonstrate that polarization-dependent
X-ray absorption spectroscopy in combination with a simulation of the spectra is able to shed a brighter light
on the spin and orbital ground state of the transition-metal ion in CoPc. Our results reveal a temperaturedependent
ground state and emphasize the importance of taking 3d correlation effects properly into account.
J. Phys. Chem. A 113, 8917 (2009)
Engineering of the Energy Level Alignment at Organic Semiconductor Interfaces by Intramolecular Degrees of Freedom: Transition Metal Phthalocyanines
We have determined the energy level alignment at interfaces between various transition metal phthalocyanines
(MnPc, FePc, CoPc, NiPc, CuPc, ZnPc) and gold using photoemission spectroscopy. We demonstrate that
the energy level alignment at these interfaces depends upon the type of transition metal. This offers a route
to adjust the hole injection barrier via the choice of otherwise equivalent molecular organic semiconductors.
In particular, the interfaces MnPc/Au and CoPc/Au are characterized by a small hole injection barrier, which
would be advantageous for applications.
J. Phys. Chem. C 113, 13219 (2009)
Full electronic excitation spectrum of condensed manganese phthalocyanine
The electronic excitation spectrum of manganese phthalocyanine films has been studied using a combination
of optical absorption and electron energy-loss spectroscopy. These studies reveal the existence of a
low lying excitation at 0.5 eV, which has not been identified previously. These excitations are localized
and have a predominant dipole allowed character. Most likely, they arise from transitions between states
which have a ligand as well as manganese metal character.
Chem. Phys. Lett. 469, 121 (2009)
People:
Benjamin Mahns
Friedrich Roth
Susi Wintz
Mandy Grobosch
Victor Aristov
Danny Haberer
Martin Knupfer
IFF / Electronic and optical Properties || Last update 12. January 2007 || Author: M. Knupfer