Nature Nanotechnology

A Gd@C82 single-molecule electret

Electrets are a class of materials that can be compared to permanent magnets. They can be used for information storage, as well as for static earphones and microphones. It has long – lasting properties. The feature of electret was first discovered by Gray in 1732. In 1892, Heaviside combined electr-et (electret) with the phrase electric and magnet firstly, and clearly put forward the concept of electret⁵.

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Fig.1 Single-electron transport of the Gd@C82 SMD

Specifically, they created a gap of about 1 nm on a 50 nm wide metal wire by using the electromigration at a low temperature of 1.6 K (about -271.6 ℃), and successfully constructed several Gd@C82 single-molecule devices (as shown in Fig. 1a). Then a source-drain voltage value was set close to zero (2 mV) . By changing the gate voltage Vg and recording the source-drain current I_ds at different gate voltage, two sets of spectral lines are obtained, corresponding to two device states (state 1 and state 2). As shown in Fig. 1b, these two states can be switched between each one by changing the gate voltage, showing two sets of different transport characteristics in the same single molecule device.

Fig.2 Density functional theory calculations revealing the SME physics

These two states probably correspond to two molecular configurations, but this configuration change is difficult to be directly observed by experiments, by using the first-principles calculation, it was found that Gd atom in the Gd@C82 molecule was located at the two most stable adjacent adsorption sites on the C82 cage, with an energy difference of ~ 6 meV (Fig. 2a). It can be seen that the positive and negative charge centers of Gd@C82 molecules do not coincide, that is, the molecule has a non-zero electric dipole moment.

Gd atom moves between two stable adsorption sites, which can change the direction of the electric dipole moment , so that the relative stability of the two adsorption sites can be regulated by the electric field. The calculation shows that Gd atom can move between the two sites under the electric field as long as the energy barrier of ~11 meV is overcome (Fig. 2). we can flip of the electric dipole moment at the level of a single molecule, i.e. the device is a monatomic (Gd) information memory.