64 research outputs found
Measuring many-body effects in carbon nanotubes with a scanning tunneling microscope
Electron-electron interactions and excitons in carbon nanotubes are locally
measured by combining Scanning tunneling spectroscopy and optical absorption in
bundles of nanotubes. The largest gap deduced from measurements at the top of
the bundle is found to be related to the intrinsic quasi-particle gap. From the
difference with optical transitions, we deduced exciton binding energies of 0.4
eV for the gap and 0.7 eV for the second Van Hove singularity. This provides
the first experimental evidence of substrate-induced gap renormalization on
SWNTs
Symmetry-selected spin-split hybrid states in C/ferromagnetic interfaces
The understanding of orbital hybridization and spin-polarization at the
organic-ferromagnetic interface is essential in the search for efficient hybrid
spintronic devices. Here, using first-principles calculations, we report a
systematic study of spin-split hybrid states of C deposited on various
ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and
hcp-Co(0001). We show that the adsorption geometry of the molecule with respect
to the surface crystallographic orientation of the magnetic substrate as well
as the strength of the interaction play an intricate role in the
spin-polarization of the hybrid orbitals. We find that a large
spin-polarization in vacuum above the buckyball can only be achieved if the
molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore
bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates.
Spin-polarized scanning tunneling spectroscopy measurements on single C
adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the
substrate and of the molecular conformation have a strong influence on the
induced spin polarization. Our finding may give valuable insights for further
engineering of spin filtering devices through single molecular orbitals.Comment: 10 pages, 9 figure
Localized state and charge transfer in nitrogen-doped graphene
Nitrogen-doped epitaxial graphene grown on SiC(000?1) was prepared by
exposing the surface to an atomic nitrogen flux. Using Scanning Tunneling
Microscopy (STM) and Spectroscopy (STS), supported by Density Functional Theory
(DFT) calculations, the simple substitution of carbon by nitrogen atoms has
been identified as the most common doping configuration. High-resolution images
reveal a reduction of local charge density on top of the nitrogen atoms,
indicating a charge transfer to the neighboring carbon atoms. For the first
time, local STS spectra clearly evidenced the energy levels associated with the
chemical doping by nitrogen, localized in the conduction band. Various other
nitrogen-related defects have been observed. The bias dependence of their
topographic signatures demonstrates the presence of structural configurations
more complex than substitution as well as hole-doping.Comment: 5 pages, accepted in PR
Change of cobalt magnetic anisotropy and spin polarization with alkanethiolates self-assembled monolayers
International audience; We demonstrate that the deposition of a self-assembled monolayer of alkanethiolates on a 1 nm thick cobalt ultrathin film grown on Au(111) induces a spin reorientation transition from in-plane to out-of-plane magnetization. Using ab initio calculations, we show that a methanethiolate layer changes slightly both the magnetocrystalline and shape anisotropy, both effects almost cancelling each other out for a 1 nm Co film. Finally, the change in hysteresis cycles upon alkanethiolate adsorption could be assigned to a molecular-induced roughening of the Co layer, as shown by STM. In addition, we calculate how a methanethiolate layer modifies the spin density of states of the Co layer and we show that the spin polarization at the Fermi level through the organic layer is reversed as compared to the uncovered Co. These results give new theoretical and experimental insights for the use of thiol-based self-assembled monolayers in spintronic devices
Selective control of molecule charge state on graphene using tip-induced electric field and nitrogen doping
The combination of graphene with molecules offers promising opportunities to achieve new functionalities. In these hybrid structures, interfacial charge transfer plays a key role in the electronic properties and thus has to be understood and mastered. Using scanning tunneling microscopy and ab initio density functional theory calculations, we show that combining nitrogen doping of graphene with an electric field allows for a selective control of the charge state in a molecular layer on graphene. On pristine graphene, the local gating applied by the tip induces a shift of the molecular levels of adsorbed molecules and can be used to control their charge state. Ab initio calculations show that under the application of an electric field, the hybrid molecule/graphene system behaves like an electrostatic dipole with opposite charges in the molecule and graphene sub-units that are found to be proportional to the electric field amplitude, which thereby controls the charge transfer. When local gating is combined with nitrogen doping of graphene, the charging voltage of molecules on nitrogen is greatly lowered. Consequently, applying the proper electric field allows one to obtain a molecular layer with a mixed charge state, where a selective reduction is performed on single molecules at nitrogen sites. The local gating applied by a tip induces a shift of the energy levels of molecules adsorbed on graphene. A team led by Jerome Lagoute at Universite Paris Diderot investigated the interplay between the charge state of molecules on pristine and doped-graphene, and the tip-induced electric fields in scanning tunneling microscopy experiments. The tip-induced electric field was found to shift the molecular levels of tetracyanoquinodimethane molecules on graphene, leading to a change of charge state at negative bias. Ab initio calculations indicated that the molecule-on-graphene hybrid structure can be regarded as an electrostatic dipole, hence the charge transfer and associated electronic charge in the molecule and graphene could be tuned by the electric field. Furthermore, inserting nitrogen atom dopants allowed shifting the energy levels of single molecules absorbed directly on the electron-donating point defects
Giant tunnel-electron injection in nitrogen-doped graphene
International audienceScanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on SiC(000¯1) and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunneling channel. At nitrogen sites, this feature vanishes due to an increase of the elastic channel that is allowed because of symmetry breaking induced by the nitrogen atoms. A large conductance enhancement by a factor of up to 500 was measured at the Fermi level by comparing local spectroscopy at nitrogen sites and at carbon sites. Nitrogen doping can therefore be proposed as a way to improve tunnel-electron injection in graphene
Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer
Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers
Higher-Indexed Moiré Patterns and Surface States of MoTe2/Graphene Heterostructure Grown by Molecular Beam Epitaxy
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
Identification and manipulation of defects in black phosphorus
We identify and manipulate commonly occurring defects in black phosphorus, combining scanning tunneling microscopy experiments with density functional theory calculations. A ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, is shown to arise from a substitutional Sn impurity in the second sublayer. Another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.We acknowledge the Indo-French Centre for the Promotion of Advanced Scientific Research (CEFIPRA) for funding our collaboration. We are especially grateful for assistance provided by the Late Ms. A. Sathidevi. We are grateful to the ANR and MOST (DEFINE2D Project No. ANR-20-CE09-0023, MOST 110-2923-M-002-010) for financial support and the TUE-CMS, JNCASR, and the Sheikh Saqr Laboratory, JNCASR, for computational resources. D.S. acknowledges support from the NPDF of the Science and Engineering Research Board (SERB), Government of India.Peer reviewe
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