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Nanoscopic functional cage clusters

Leonid Kliuienko, Anna Strozecka, Bert Voigtländer

Over the last few years the area of single-molecule science has drawn an overall attention. Among the studied molecules the buckminster fullerenes (C60) are considered to be especially attractive, due to their interesting features and possible applications in molecular electronics. Understanding of their properties in different environments is crucial for predicting the behaviour of the fullerene-based molecular devices.

Our studies of molecular systems are performed using a low temperature scanning tunneling microscope (STM) operating at 6K in order to exploit the ability to:

  1. Build and modify molecular structures molecule by molecule
  2. Perform electronic and vibration spectroscopy at the single molecule level

Conductance of Single Endofullerene Molecules

Miniaturization of electronic devices down to the atomic and molecular size requires understanding of the properties of nanoscale contacts. Scanning tunnelling microscopy (STM) allows to study molecular contacts with defined junction geometry. The structure of the investigated system can be imaged prior to and after contact formation. Therefore the identity, location, and number of atoms/molecules in between the electrodes can be well controlled.

Endohedral fullerenes are fullerenes with one ore more (metal) atoms inside the fullerene cage. The conductance of a single endohedral fullerene upon contact with the STM tip is compared to that of the hollow C60. Dimetallofullerenes have additional degrees of freedom associated with the two encapsulated atoms that can influence the electron transport trough the Ce2@C80 molecule.
The sample has been prepared by the co-deposition of Ce2@C80 and C60 on the Cu(111) substrate in order to have both Ce2@C80 and C60 reference next to each other on the sample. Figure below shows an STM image of the Cu(111) surface covered Ce2@C80 and C60 molecules. The C60 can be easily distinguished from the endohedral complexes. Ce2@C80 molecules appear in the image as smooth and featureless protrusions, whereas C60 is characterized by a clear three-lobed pattern. The three-fold symmetry allows also to establish that C60 molecules are adsorbed with a hexagonal face pointing up.

STM Image of CE2STM image of Ce2@C80 and C60 molecules co-deposited on Cu(111). C60 (indicated by arrows) can be distinguished from endohedral fullerenes due to the characteristic internal structure.

Typical conductance curves obtained for C60 and Ce2@C80 fullerenes are shown in the next figure. The zero position of the tip is defined by the feedback parameters: I0=3nA and V0=300mV, afterwards tip is moved by z = 6 - 6.5 Å towards the molecule. The contact process has been completely reversible. The hysteresis free traces for approach and retraction show that neither tip nor the molecule has changed during the measurements, what has been confirmed by STM images and dI/dV spectra.

Conductance curves obtained for C60Conductance curves obtained for C60 (blue) and Ce2@C80 molecules (red), as a function of tip-molecule distance.

The fact that the two jumps in conductance observed for the endohedral fullerene occur as well in the curves for the undoped molecule suggests that none of the changes can be unambiguously assigned to the switching of the geometry of the encaged metal atoms.
The first evident difference between the results for empty and doped fullerene is the range of conductance of the two molecules after the contact formation. The tunneling-to-contact transition occurs for both fullerenes at roughly the same tip displacement, however, Ce2@C80 exhibits about five times lower conductance than C60. The explanation is not simply related to the different sizes of the carbon cages but may be related to the presence of the metal atoms. The lowest unoccupied orbitals of Ce2@C80 are dominated by cerium states and it can be expected that the partial localization of the orbital on metal atoms disturbs the conduction mechanism.

The last characteristic feature in the curves is the second increase in conductance which for both molecules takes place at the tip position z ~ - 4.5 Å. For C60 this rise is actually a discontinuous jump, while in case of Ce2@C80 the transition is smooth. The origin of this conductance change is not exactly clear. It is most probably due to a rearrangement of the junction geometry, which leads to the higher number of available conductance channels. The fact that for exactly the same tip different behaviour is observed for hollow and doped fullerene indicates that the rise in conductance is related to the molecule itself. The possible explanations are e.g. a reversible modification of molecular geometry or the formation of additional tip-fullerene bonds.

Lateral manipulation of single C60

The stability of the LT-STM allows to use the microscope as a tool to manipulate single C60 molecules. On the small tip-sample distances the chemical forces between the tip and the molecule start to play a significant role. When the tip is moved in the lateral direction, the molecule is pushed or pulled along the surface, due to attractive or repulsive interaction with the tip. The pushing or pulling mode can be recognized from the record of the tip height during the manipulation. The results of the manipulation of C60 on Ag(100) are presented in the figure below. By controlling the lateral position of the molecules on the surface one can create artificial structures. The "NANO" shown on the figure was built by manipulating single C60. The size of each letter is about 16×16 nm.

Assembly of the artificial structure using the lateral manipulation technique. The structure "NANO" assembled from single C60 on Ag(100). Assembly of the artificial structure using the lateral manipulation technique. The structure "NANO" assembled from single C60 on Ag(100).

The tip height curve (see figure) corresponds to a signature expected for pulling of C60 along the silver surface. The molecule is attracted by the tip and follows it, until the hopping by one adsorption site occurs. This is seen as a sudden upward jump in the tip height. Afterwards the tip moves down the contour of the molecule and the whole process repeats. The periodicity in the height plot reflects the periodicity of the substrate lattice.

Lateral ManipulationTip height contour during the manipulation of C60 on Ag(100). The length of a single hops is 2.9 Å and corresponds to the nearest neighbour distance.

Electronic and vibronic properties of C60 on Cu(111)

A low temperature scanning tunneling microscope is a powerful tool for studying and controlling the properties of single molecules with high precision. STM spectroscopy techniques provide a detailed insight into the electronic and vibrational properties of the molecules, combined with atomic resolution in the real space.

We made the LT-STM spectroscopic study of C60 on metal substrates: Cu(111) and Ag(100). The STS spectra (Scanning Tunneling Spectroscopy), show features attributed to HOMO, LUMO and LUMO+1 molecular states. Information about vibronic properties is obtained from IETS spectra (Inelastic Electron Tunneling Spectroscopy).
The electronic spectra measured in STM allow to trace the interaction of C60 with the substrate. The figure below shows (dI/dV)/(I/V) spectrum of C60 on Cu(111). Four main features appear in the plot: a broad peak at 1.8 V (LUMO+1), two peaks at 0.55 V and -0.14 V (LUMO) and a peak at -1.7 V (HOMO). These features can be attributed to the HOMO, LUMO and LUMO+1 molecular states of a neutral molecule. The broadening of the peaks arises from the interaction with the substrate. The charge transfer from the substrate to the fullerene cage results in the partial occupation of the LUMO state and splitting of the orbital into two peaks, one of them lying below the Fermi level, in the filled-states region of the spectrum.

Figur 1Normalized dI/dV spectrum of C60 on Cu(111) at 6K. The spectrum was taken at the spot indicated on the inset image. Grey curve was obtained on the bare Cu(111) substrate. The specified peaks correspond to the molecular orbitals of a free C60.

Information about the vibrational modes of the molecule can be extracted from the d2I/dV2 plot. The peak in the positive voltage, together with symmetrically lying dip in the negative voltage, is the signature of the inelastic excitation of a vibrational mode. The following figure presents IETS results obtained for C60 on Cu(111).

Figur 2IETS vibrational spectrum of a single C60 molecule on Cu(111). The grey curve corresponds to spectrum taken on the bare copper substrate. Two indicated peaks (with corresponding dips in the negative voltages) are signature of the molecular vibrations.

Two observed features are attributed to molecular vibrations: peak (dip) around 53 meV and peak (dip) around 138 meV. The vibrational structure of a free C60 shows 46 differenent vibrational modes, with energies between 33 and 195 meV. Most of them are highly degenerate, due to icosahedral symmetry of the cage, and give together 174 intrernal vibrations. Only one of the 46 vibrations has energy around 53 meV and can be assigned to the lower peak. It is the Hg(ω2) mode (energy 56 meV), that corresponds to the "gerade" breathing of the carbon cage. We attribute the second peak to the Hg(ω5) mode of energy 141 meV. Both of the modes are Raman active, and their energies are good determined by experiment and theory.