Several research groups in last years have investigated the electrical bistability of organic films. Indeed some molecular films have the particular properties that the current-voltage electrical characteristic can switch from a low conduction state to a high conduction state when a voltage threshold Vth1 is reached. We have electrical bistability when the switch is reversible in the sense that the electrical characteristic commutates back to the original state of conduction for a voltage Vth2. This suggests it might be possible to demonstrate and to build up high-density memory devices based on bistable molecular films.
Until present days several mechanisms have been proposed to explain and understand the electrical bistability of molecular films. The proposed mechanisms can be resumed as follows:
- the electrical external field (bias), induces a redox equilibrium between two states: the two redox states inside bandgap have different alignments with respect to the electrodes work function and the electrical response is due to different injection or extraction probability of hole/electron observed for the two states;
- the electrical external field, induces a charge transfer of electrons from one of the electrodes to the molecules increasing the conjugation;
- the organic material acts as an insulating layer in which a conductive path can be generated by one-dimensional “filament” connecting some conductive elements by quantum tunneling: the conductive elements can be either metal ions diffused from electrodes or oxygen vacancies;
- the mechanism of switching is an interface effect between organic layer and top electrode (a metal) where the resistive switching ratio depends on the nature of the oxide and layer thickness. Indeed binary oxide layers can be electrically bistable.
In the first two models the resistive switching is an intrinsic effect due to the properties of the organic material, while for the mechanisms 3. and 4. the resistive switching is an estrinsic effect due either to the electro-formed filament or to the oxide at the metal electrode-organic layer interface.
In particular, in STMicroelectronics we have investigated the electrical bistability of spin cast films of halogenated fluorescein dyes. In the experiments, the fluorescein dyes organic film is spin cast on ITO or ZnO substrate and aluminum top electrode is evaporated. In particular an ON/OFF ratio as high as 350,000 has been reported for electrical current of the rose bengal (halogenated fluorescein) films, attributed to the very low OFF current, also compared to the OFF current of the fluorescein (not halogenated fluorescein) films indeed with a quite low ON/OFF ratio.
The investigated molecular structures of the investigated fluorescein dyes are shown in figure 1. We calculated the electronic properties of the single molecules within the time dependent density functional theory (TD-DFT) approach. For the modeling of the band structure of the organic crystals, we used the density functional theory (DFT).
Our conclusions have been that for the crystals the erythrosine B (halogenated) and fluorescein (not halogenated) salt have similar bandwidths, so that halogenated fluorescein dyes including xanthene rings with halogen substituent atoms have resistivity comparable to that of fluorescein salt without halogen atoms. Therefore, the improved ON/OFF ratio of halogenated fluorescein dyes with halogen substituent atoms would be explained in terms of hetero-interface effects rather than in terms of the internal arrangement of the molecules in the crystal.
Therefore we applied the time dependent density functional theory (TD-DFT) approach in order to understand the chemical processes that could be implicated in the formation of a metal oxide layer at the top electrode. Indeed, it has been observed that binary oxide layers can be electrically bistable (mechanism 4.).
The rose bengal showed high quantum yield of the triplet state and, as a result, rose bengal has very strong oxidation properties. Indeed, triplet state could react either by electron-transfer producing free radicals that interacted with oxygen to form active oxygen species or via energy transfer process that generates singlet oxygen. On the other hand, the high electronic affinity of rose bengal confirmed the elevated tendency to reduce itself. Those are oxidation mechanisms able to support the formation of electrically bistable thin oxide layer at the organic layer/electrode interface. The ON/OFF ratio dependeds on the nature of the oxide and layer thickness. Lactone form of fluorescein was characterized by low triplet yield that made films based on this molecule disadvantaged in the formation of oxide layers.
It was found that rose bengal is the molecule more prone to promote the formation of an aluminum oxide layer at the electrode giving rise to electrical bistability via the mechanism 4.
The full paper where we investigated the electrical bistability of fluorescein dye films is:
“New insights into oxidation properties and band structure of fluorescein dyes from ab initio calculations” – F. Buonocore, A. di Matteo, Theoretical Chemistry Accounts 131, 1130 (2012).