Charge transport in a single molecule transistor probed by scanning tunneling microscopy

We report on the scanning tunneling microscopy/spectroscopy (STM/STS) study of cobalt phthalocyanine (CoPc) molecules deposited onto a back-gated graphene device. We observe a clear gate voltage ( V g ) dependence of the energy position of the features originating from the molecular states. Based on the analysis of the energy shifts of the molecular features upon tuning  V g , we are able to determine the nature of the electronic states that lead to a gapped differential conductance. Our measurements show that capacitive couplings of comparable strengths exist between the CoPc molecule and the STM tip as well as between CoPc and graphene, thus facilitating electronic transport involving only unoccupied molecular states for both tunneling bias polarities. These findings provide novel information on the interaction between graphene and organic molecules and are of importance for further studies, which envisage the realization of single molecule transistors with non-metallic electrodes

Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene

Molecular electronics is based on the use of single molecules as elementary functional electronic components. It is considered to be a promising strategy for further miniaturization of electronic circuits and long-term replacement of semiconductor technology. Moreover the quantum mechanical nature of molecules may be used to design new logical functions, beyond the electron-charge based data processing of classical transistors. With the experimental probing and manipulation of electron and nuclear spin, and the observations of quantum interferences in such systems, this area of research has gained increasing attention in the last decade.Contacting individual molecules in order to perform transport experiments at such a small scale is challenging. Various methods based on the creation of metallic nano-contacts into which molecules are deposited have shown promising results, and already lead to the realization of single-molecule transistors.On the other hand, scanning tunneling microscopy(STM) has also proven to be a successful method for studying single molecules on surfaces and probing some of their electronic properties. Due to geometrical constraints, it is extremely challenging to implement a third gate electrode in such an experiment. This is, however, a prerequisite for controlling the energy of the molecular level responsible for charge transport. The aim of this work was to use graphene on SiO_2/Si as a substrate, Si and graphene being independently electrically contacted and serving as drain- and gate-electrode. The very low density of states of graphene makes it screen only a fraction of the electric field generated by the applied potential on the silicon. A molecule deposited on the graphene is thus placed in a controllable electric field, so that the geometrical arrangement STM tip/molecule/graphene/SiO_2/Si forms a molecular transistor,which makes it possible to tune the molecular levels contributing to charge transport in an STM.A newly acquired low temperature STM was first put into operation. Spatial and energy resolution of the device could be tested, before graphene deposited on various substrates was examined. This consists in a preliminary work laying the basis for the later deposition of molecules onto graphene. STM spectroscopy could furthermore demonstrate the emergence of Landau levels in graphene on NbSe_2 when a magnetic field is applied.The molecules investigated in this work are cobalt phthalocyanines (CoPc). They are planar organic molecules, which have found application in various areas. Since they are thermally stable, they can be easily deposited on various substrates and belong to the most studied molecules in surface science. The adsorption of CoPc molecules on metallic surfaces has been studied first, showing a strong interaction between the molecular orbitals and the substrate, leading to a charge transfer. The insertion of graphene between metallic substrate and molecule was then investigated as a possible buffer layer for decoupling of the molecular levels. STM spectroscopy studies indicate that the molecular energy levels of CoPc on graphene do in fact correspond to the ones of the gas phase. Finally, CoPc molecules on graphene on SiO_2 were studied and their electronic properties probed as a function of the potential applied to the gate. A control of the energy of molecular states contributing to the tunneling current could be achieved, with an efficiency similar to classical transport experiments. The possibility to combine the charge transport characterization of molecules by means of a gate electrode with the spatial resolution of a STM was thus demonstrated. This opens a door towards promising future experiments with a combined control over the spatial position of the source electrode in the picometer-range and over the electronic properties of the studied nanostructures

Fundamental quantum noise mapping with tunnelling microscopes tested at surface structures of subatomic lateral size

We present a measurement scheme that enables quantitative detection of the shot noise in a scanning tunnelling microscope while scanning the sample. As test objects we study defect structures produced on an iridium single crystal at low temperatures. The defect structures appear in the constant current images as protrusions with curvature radii well below the atomic diameter. The measured power spectral density of the noise is very near to the quantum limit with Fano factor F = 1. While the constant current images show detailed structures expected for tunnelling involving d-atomic orbitals of Ir, we find the current noise to be without pronounced spatial variation as expected for shot noise arising from statistically independent events

Rashba splitting of graphene-covered Au(111) revealed by quasiparticle interference mapping

We report on low-temperature scanning tunneling spectroscopy measurements on epitaxial graphene flakes on Au(111). We show that using quasiparticle interference (QPI) mapping, we can discriminate between the electronic systems of graphene and Au(111). Beyond the scattering vectors, which can be ascribed to the elastic scattering within each of the systems, we observe QPI features related to the scattering process between graphene states and the Au(111) surface state. This additional interband scattering process at the graphene/Au(111) interface allows the direct quantitative determination of the Rashba-splitting of the Au(111) surface state, which cannot be evaluated from QPI measurements on pure Au(111). This experiment demonstrates a unique local spectroscopic approach to investigate the Rashba-split bands at weakly interacting epitaxial graphene/substrate interfaces

Nucleation and growth of nickel nanoclusters on graphene Moiré on Rh (111)

Regularly sized Ni nanoclusters (NCs) have been grown on a graphene Moiré on Rh(111). Using scanning tunneling microscopy, we determine that initial growth of Ni at 150 K leads to preferential nucleation of monodispersed NCs at specific sites of the Moiré superstructure. However, a defined long-range ordering of NCs with increasing coverage is not observed. Room temperature Ni deposition leads to the formation of flat triangular-shaped islands which are well-matched to the Moiré registry

The local magnetic properties of [MnIII6 CrIII]3+ and [FeIII6 CrIII]3+ single-molecule magnets deposited on surfaces studied by spin-polarized photoemission and XMCD with circularly polarized synchrotron radiation

It is demonstrated that local magnetic moments of single molecule magnets (SMM) normally studied by XMCD at very low temperatures and high magnetic fields can be measured by means of spin-resolved electron emission in the paramagnetic phase at room temperature by use of circularly polarized radiation.publishe

Exposure of [(Mn6CrIII)-Cr-III](3+) single-molecule magnets to soft X-rays: The effect of the counterions on radiation stability

Helmstedt A, Sacher M, Gryzia A, et al. Exposure of [(Mn6CrIII)-Cr-III](3+) single-molecule magnets to soft X-rays: The effect of the counterions on radiation stability. Journal of Electron Spectroscopy and Related Phenomena. 2012;184(11-12):583-588.X-ray absorption spectroscopy studies of the [(Mn6CrIII)-Cr-III](3+) single-molecule magnet deposited as a microcrystalline layer on gold substrates are presented. The oxidation state of the manganese centers changes from Mn-III to Mull due to irradiation with soft X-rays. The influence of the charge-neutralizing anions on the stability of [(Mn6CrIII)-Cr-III](3+) against soft X-ray exposure is investigated for the different anions tetraphenylborate (BPh4-), lactate (C3H5O3-) and perchlorate (ClO4-). The exposure dependence of the radiation-induced reduction process is compared for [(Mn6CrIII)-Cr-III](3+) with the three different anions. (C) 2011 Elsevier B.V. All rights reserved

<i>In Situ</i> Fabrication Of Quasi-Free-Standing Epitaxial Graphene Nanoflakes On Gold

Addressing the multitude of electronic phenomena theoretically predicted for confined graphene structures requires appropriate <i>in situ</i> fabrication procedures yielding graphene nanoflakes (GNFs) with well-defined geometries and accessible electronic properties. Here, we present a simple strategy to fabricate quasi-free-standing GNFs of variable sizes, performing temperature programmed growth of graphene flakes on the Ir(111) surface and subsequent intercalation of gold. Using scanning tunneling microscopy (STM), we show that epitaxial GNFs on a perfectly ordered Au(111) surface are formed while maintaining an unreconstructed, singly hydrogen-terminated edge structure, as confirmed by the accompanying density functional theory (DFT) calculations. Using tip-induced lateral displacement of GNFs, we demonstrate that GNFs on Au(111) are to a large extent decoupled from the Au(111) substrate. The direct accessibility of the electronic states of a single GNF is demonstrated upon analysis of the quasiparticle interference patterns obtained by low-temperature STM. These findings open up an interesting playground for diverse investigations of graphene nanostructures with possible implications for device fabrication

Spin resolved photoelectron spectroscopy of [(Mn6CrIII)-Cr-III](3+) single-molecule magnets and of manganese compounds as reference layers

Helmstedt A, Müller N, Gryzia A, et al. Spin resolved photoelectron spectroscopy of [(Mn6CrIII)-Cr-III](3+) single-molecule magnets and of manganese compounds as reference layers. Journal of Physics Condensed Matter. 2011;23(26): 266001.Properties of the manganese-based single-molecule magnet [(Mn6CrIII)-Cr-III](3+) are studied. It contains six Mn-III ions arranged in two bowl-shaped trinuclear triplesalen building blocks linked by a hexacyanochromate and exhibits a large spin ground state of S-t = 21/2. The dominant structures in the electron emission spectra of [(Mn6CrIII)-Cr-III](3+) resonantly excited at the L-3-edge are the L3M2,3M2,3, L3M2,3V and L3VV Auger emission groups following the decay of the primary p(3/2) core hole state. Significant differences of the Auger spectra from intact and degraded [(Mn6CrIII)-Cr-III](3+) show up. First measurements of the electron spin polarization in the L3M2,3V and L3VV Auger emission peaks from the manganese constituents in [(Mn6CrIII)-Cr-III](3+) resonantly excited at the L-3-edge near 640 eV by circularly polarized synchrotron radiation are reported. In addition spin resolved Auger electron spectra of the reference substances MnO, Mn2O3 and Mn-II(acetate)(2)center dot 4H(2)O are given. The applicability of spin resolved electron spectroscopy for characterizing magnetic states of constituent atoms compared to magnetic circular dichroism (MCD) is verified: the spin polarization obtained from Mn-II(acetate)(2)center dot 4H(2)O at room temperature in the paramagnetic state compares to the MCD asymmetry revealed for a star-shaped molecule with a (Mn4O6)-O-II core at 5 K in an external magnetic field of 5 T