Li@C60 as a multi-state molecular switch

Abstract

The field of molecular electronics aims at advancing the miniaturization of electronic devices, by exploiting single molecules to perform the function of individual components. A molecular switch is defined as a molecule that displays stability in two or more states (e.g. “on” and “off” involving conductance, conformation etc.) and upon application of a controlled external perturbation, electric or otherwise, undergoes a reversible change such that the molecule is altered. Previous work has shown multi-state molecular switches with up to four and six distinct states. Using low temperature scanning tunnelling microscopy and spectroscopy, we report on a multi-state single molecule switch using the endohedral fullerene Li@C 60 that displays 14 molecular states which can be statistically accessed. We suggest a switching mechanism that relies on resonant tunnelling via the superatom molecular orbitals (SAMOs) of the fullerene cage as a means of Li activation, thereby bypassing the typical vibronic excitation of the carbon cage that is known to cause molecular decomposition.

Sublimation of Li@C60

“…This value is remarkably close to the activation energy determined previously for Li loss from the dimerised endohedral species in solution (1.1 ± 0.2 eV) [15]. It is much lower that the value of 5.4±0.2 eV obtained for release of Li from the isolated gas phase cationic endohedral molecules that is similar to the threshold energy for Li + implantation in C 60 [16] and the bias voltage at which Li loss from the C 60 cage is observed in STM experiments [9]. The determined activation energy for the material within the oven is thus not a consequence of a simple unimolecular decay reflecting the loss of Li from an intact C 60 cage but is a consequence of more complex reactions with impurity species in the oven.…”

“…Laser-desorption FTICR mass spectrometry of the starting material in both the positive and negative mode showed only minor traces of C 60 with the non-fullerene content being predominantly the stabilising anions and solvent residues. As we have shown previously [8,9] neutral Li@C 60 is evaporated from the oven along with impurities and some C 60 . The material is typically heated to a temperature of ca.…”

“…In our previous studies of scanning tunnelling spectroscopy of Li@C 60 deposited on Au(1 1 1) [8,9] we showed that it was possible to distinguish Li@C 60 from C 60 by imaging the molecules at a bias voltage of +2.5 V. The thermal treatment of the endohedral fullerene material prior to deposition is similar to that used for the gas phase experiments (to ensure that deposition of non-fullerene material from the oven is minimised) and this leads to a mixture of Li@C 60 and C 60 being deposited on the gold surface. The fullerene molecules form hexagonally close-packed islands.…”

“…The fullerene molecules form hexagonally close-packed islands. At negative bias voltages (imaging the filled states) the molecules are indistinguishable, however at the positive bias voltage of +2.5 V, the S-SAMO orbital of Li@C 60 is accessed and the endohedral molecules appear to "light up" showing a diffuse circular image with no apparent structure [9]. It is therefore possible to directly count the endohedral and empty fullerenes that are deposited on the substrate for a given thermal treatment in the oven.…”

“…We have recently combined gas phase studies using mass spectrometry and photoelectron spectroscopy with scanning tunnelling microscopy to obtain insight into the socalled Super-Atom Molecular Orbital states of the molecule [8] and showed that it is possible to demonstrate multistate switching in a single Li@C 60 molecule with 14 distinct switching states [9]. These experiments required the preparation of a clean beam of Li@C 60 molecules (containing some proportion of empty C 60 ) that was prepared by heating the commercial [Li + @C 60 ]PF − 6 material in a quartz ampoule at elevated temperatures.…”