Research results 03｜Research Center for Materials Nanoarchitectonics

Single-molecule-level molecular memories for ultra-high density storage

T. Nakayama , M. Aono
Co-workers: M. Nakaya , S. Tsukamoto

Ultrahigh-density data storage has been considered to be one of the important outcomes by utilizing single-molecule manipulation with a scanning tunneling microscope (STM). However, there has been a crucial problem for many years; how to achieve reversible and repeatable control of a molecular bit to represent 0 and 1. We solved this long-standing problem by reversibly controlling bound and unbound states of C₆₀ molecules at room temperature, and demonstrated actual bit operations at a bit density of 190 Tbits/in².

Fig. 1 Schematic illustration of local and reversible control (equivalent to bit operation)of bound and unbound states of C60 molecules (top). A series of STM images showing single-molecule-level bit operation achieved by this technique

We used a thin film of fullerene C₆₀ molecules and controlled single-molecule-level chemical reaction between C₆₀ molecules in the film using an STM tip. We found that negative and positive ionization of a designated C₆₀ molecule can selectively trigger polymerization and depolymerization reactions of the designated C₆₀ molecule, respectively, with an adjacent molecule in the film. The mechanism of this STM-induced chemical reaction was experimentally and theoretically studied. When the film of C₆₀ molecules is negatively biased against an STM tip, electron donation to C₆₀ molecules occurs under the tip and the lowest unoccupied molecular orbital (LUMO) of C₆₀ , the bound state between C₆₀ molecules, is partially occupied. Further electronic excitation by tunneling electrons fulfills the bound state with two electrons, stabilizing the bound C₆₀ molecule. When using opposite biases, the bound C₆₀ molecules are positively ionized by extraction of electron. The destabilized bound state finally dissolves with the help of electronic excitation by tunneling electrons. Because electronic excitation by STM is in a very confined area, only a single C₆₀ molecule underneath the STM tip can be controlled. Bound and unbound states of C₆₀ molecules are easily recognized by the appearance and disappearance of depression of the film, and we demonstrated ultradense data storage at a bit density of 190 Tbits/in² by controlling the chemical reactions at a single-molecule precision as shown in Fig. 1.