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Carbon Nanostuctures

INTRODUCTION

Fullerenes are unique carbon structures that have great potential for uses in future nanotechnological applications. The simplest fullerene structure is the C60 molecule, which consists of 60 carbon atoms arranged in a spherical shell.

Our research has focused on studying the spatial variation of C60 electronic structure and examining how it is modified by local perturbations. The majority of our work has been conducted on C60 deposited on the Ag(100) surface at 7K.

Individual C60 molecules

Theoretical Modeling

Doped C60 - Nano Pacman

C60 Monolayer

Individual C60 Molecules

The topography of isolated C60 molecules on the Ag(100) surface is shown in Figure 1a. Each C60 molecule assumes one of four orientations. In unoccupied state images (positive sample voltage) C60 molecules appear to be composed of pentagonal segments. The appearance of the C60 topography is determined by the nature of the C60 electronic states probed by the STM tip. By changing the sign of the tunneling voltage from positive to negative, the STM switches from probing unoccupied states to probing occupied states. The occupied C60 states give the C60 molecule a “dimpled golf ball” appearance, as seen in the adjacent figure.
The electronic states that produce the C60 topography can be directly measured using scanning tunneling spectroscopy. Measuring the C60 electronic spectrum at various points on the C60 molecule reveals four main spectroscopic features, which are attributed to the C60 HOMO (Highest Occupied Molecular Orbital), split LUMO (Lowest Unoccupied Molecular Orbital), and LUMO+1. These spectroscopic measurements further show that the C60 electronic structure is highly spatially inhomogeneous.
This spatial variation is due to inhomogeneity of the C60 molecular orbitals and is best visualized by mapping the height of each spectroscopic resonance at each point on the molecule. The resulting images are shown here. These STS maps represent energy-resolved images of C60 molecular orbitals.

See references 5, 6

Theoretical Modeling

STS spectra and images contain a lot of information about the nature of C60 states, as well as the C60-Ag(100) interaction. Using density functional theory calculations we can determine the extent to which substrate-C60 bonding, charge transfer, and state hybridization work together to produce the observed electronic features.

Theoretical modeling also reveals the extent to which the C60 topography affects the way STS dI/dV maps measure C60 molecular states. The most common way to visualize theoretical maps of C60 states is to plot surfaces of constant state density (state iso-surfaces). The state iso-surfaces for the C60 LUMO and LUMO+1 states are shown in the figure below. These maps, however, due not accurately depict the STS maps. This is not surprising since STS maps do not measure state density iso-surfaces, but rather they measure local state density over topographical contours. Mimicking the C60 topography by a hemisphere and calculating state density on this contour still does not accurate reproduce the STS measurement, as seen below.

The STS dI/dV maps are well reproduced once the true C60 topography is taken into account. By calculating state density over the simulated STM tip trajectory (the theoretical topographs below), the remaining discrepancies are eliminated and a good match is achieved between theoretical and experimental maps.
For further information, see references 4, 5, 6

Doped C60 - Nano Pacman

The molecular manipulation capabilities of the STM can be used to reversibly add dopants to a single C60 molecule. This is achieved by moving individual C60 molecule over K atoms stuck on the Ag(001). Due a strong attaction between K and C60, the K atoms stick to the C60 molecule and follow it along during further manipulations. As the movie in the figure below shows, several K atoms can be attached using this technique.

Due to large electronegativity difference between K and C60, the attached K atoms transfer some of their charge to the C60, making the C60 molecule more negative. The degree of electron transfer can be estimated using scanning tunneling spectroscopy. The figure alongside shows STS spectra taken on KxC60 molecules during various stages of doping.

The shifts observed in the C60 molecular states are consistent with a charge transfer of ~0.6 electrons being added to the C60 molecule for each added K.

Please see reference 3 below for more information on this technique.

C60 Monolayer

Monolayers of C60 adsorbed on metal surfaces provide an excellent two-dimensional laboratory for exploring the complex electronic and structural phase diagrams of the fullerides. We have fabricated potassium doped C60 (KxC60) monolayers on the Au(111) surface with a wide range of K contents. Using high-resolution scanning tunneling microscopy and spectroscopy, we observed a series of novel electronic and orientational phases.

Fig. 1: STM topography (a) and spectroscopy (b) of a K3C60 monolayer

 We begin by describing our results for the x = 3 monolayer. Fig. 1a shows the STM topograph of a K3C60 monolayer which exhibits a complex  superstructure of bright C60 molecules having different orientations from their dimmed nearest neighbors.  The dI/dV spectra (Fig. 1b) of a K3C60 monolayer display a pronounced peak at the Fermi energy (EF), reflecting the large electronic density of states (DOS) of a metallic, half-filled LUMO-derived band (LUMO = Lowest Unoccupied Molecular Orbital).

Fig. 2: STM topography (a) and spectroscopy (b) of a K4C60 monolayer

 As the K content is increased to x = 4, however, the monolayer dI/dV spectra exhibit an energy gap Δ ~ 0.2 eV right at EF. This insulating gap has been demonstrated to be induced by a molecular Jahn-Teller (JT) distortion2. The JT-distortion breaks the spherical symmetry of the buckyball, and splits the degenerate LUMO orbitals into two filled lower JT-orbitals and one empty upper JT-orbital with an energy gap in between. The metal-insulator transition from K3C60 to K4C60 is accompanied by the appearance of a complex “cross-like” orientational ordering (Fig. 2a) in the K4C60 monolayer.

Fig. 3: STM topography (a) and spectroscopy (b) of a K4+dC60 monolayer

 Further K-deposition in the regime 4 < x < 4.5 induces a rearrangement of the C60 molecules that leads to a novel “pinwheel-like” local structural unit1. Fig. 3a shows a domain of seven-sublattice pinwheels forming a close-packed 7×7 superstructure. dI/dV spectroscopy (Fig. 3b) of the pinwheel molecules reveals an electronic structure nearly identical to that seen for the insulating K4C60 cross-phase. Thus the difference between the cross and pinwheel phases lies purely in the lattice structure and intermolecular orientational order.

Fig. 4: STM topography (a) and spectroscopy (b) of a K5C60 monolayer

When the K content is increased further to x ~ 5, a new stoichiometric, metallic phase emerges, as shown by the ordered domain of bright molecules in Fig. 4a. dI/dV spectra (Fig. 4b) reveal that the JT-insulating gap disappears and a finite density of states exists at EF, indicating a metallic ground state. This indicates that C60 molecules in this new phase are in a C605- charge state where the fifth electron occupies an upper JT-split band. The K5C60 molecules form a triangular lattice with a prominent 2×2 superstructure composed of bright "tri-star" features. The orientational ordering of K5C60 closely resembles that found in the metallic K3C60 monolayer
We propose that the driving force for all KxC60 orientational phases observed here is the minimization of electron kinetic energy through maximization of the overlap of relevant neighboring molecular orbitals. For the insulating pinwheel and cross-phases (4 ≤ x < 5) this takes the form of a superexchange type mechanism. While in the metallic K3C60 and K5C60 phases it is the enhancement of first order electron hopping via maximization of the overlap of occupied electron wavefunctions.

In summary, we found that KxC60 monolayers undergo a reentrant metal-insulator-metal transition as x is varied from 3 to 5. The insulating phase in K4C60 was demonstrated to be induced by a molecular Jahn-Teller distortion. Each electronic phase has a novel orientational ordering associated with it. This includes a highly complex, seven-sublattice pinwheel orientational structure in the insulating K4+δC60 phase. These results highlight the intricate interplay between the various degrees of freedom in the fullerides.

References:

Our related publications:

  1. Novel Orientational Ordering and Reentrant metallicity in KxC60Monolayers for 3<x<5
    Yayu Wang, R. Yamachika, A. Wachowiak, M. Grobis, K. H. Khoo, D.-H. Lee, Steven G. Louie, M. F. Crommie, Phys. Rev. Lett 99, 086402 (2007)

  2. Visualization of the molecular Jahn-Teller effect in an insulating K4C60 monolayer.
    A. Wachowiak, R. Yamachika, K. H. Khoo, Y. Wang, M. Grobis, D.-H. Lee, Steven G. Louie, M. F. Crommie, Science 310, 468 (2005)

  3. Controlled atomic doping of a single C60 molecule
    R. Yamachika, M. Grobis, A. Wachowiak, and M. F. Crommie
    Science 304, 281 (2004). Published online 11 March 2004 (10.1126/science.1095069)

  4. Charge transfer and screening in individual C60 molecules on metal substrates: a scanning-tunneling-spectroscopy and theoretical study
    Xinghua Lu, M. Grobis, K.H. Khoo, Steven G. Louie, and M. F. Crommie
    Phys. Rev. B 70, 115418 (2004)

  5. Energy resolved imaging of fullerene molecular orbitals
    M. Grobis, Xinghua Lu, K.H. Khoo, Steven G. Louie, and M. F. Crommie
    Proceedings of the 12th International Conference on Scanning Tunneling Microsocopy/Spectroscopy and Related Techniques. AIP CP# 696, 20 (2003)

  6. Spatially mapping the spectral density of a single C60 molecule
    Xinghua Lu, M. Grobis, K.H. Khoo, Steven G. Louie, and M. F. Crommie
    Phys. Rev. Lett. 90, 096802 (2003)

  7. Local electronic properties of a molecular monolayer: C60 on Ag(100)
    M. Grobis, X. Lu, and M. F. Crommie
    Phys. Rev. B 66, 161408 (2002)

© Crommie Group 2007. All Rights Reserved