Bottom-Up Approach to Innovative Memory Devices: I. Intrinsic and Environmental Effects on the Molecular Component

Abstract

The 50 nm-thick polystyrene (PS) film, involved in some innovative memory devices, contains 8-hydroxyquinoline (8HQ) molecules and gold nanoparticles. A model where molecular localized properties directly reflect on macroscopic behavior of a complex system has been tested in the present work, which is focused on the structural and electronic properties of the 8HQ-PS mixture modeled in a continuum scheme: one 8HQ molecule with a polarizable continuum model (PCM) whose reliability has been checked by comparison with periodic DFT calculations of 8HQ-PS crystalline structures. A comprehensive study of the keto-enolic tautomerization of 8HQ has been performed, at the DFT level using B3LYP, LC-PBE, and M052X functionals and a polarized double-ζ basis set. The energetics of the obtained structures (minima and transition states) have been refined by single point calculations at the CCSD(T) level with the aug-cc-pVDZ basis set. Our calculations predict the enolic tautomer to be the most stable for the isolated and PS-solvated 8HQ in its neutral form, with a tautomerization barrier much larger than the thermal energy at the working conditions. The opposite trend has been found for the charged (both positive and negative) 8HQ, with ketonic tautomers being the most stable. In a first approximation of weak interaction with the aluminum electrodes, the electricfield effects have also been taken into account for the calculations of electron affinities and ionization potentials of 8HQ molecules. The electron and hole injection barriers issuing from these results are in good agreement with the experimental observations.

Photophysical Properties of 8-Hydroxyquinoline-5-sulfonic Acid as a Function of the pH: A TD-DFT Investigation

“…These values are in better agreement also with those reported by Barone et al for the ground state tautomerization of the parent 8-HQ molecule using hybrid functionals. 42 Nevertheless, both the barrier and the stabilization of the tautomers are still underestimated.…”

Bottom-Up Approach to Innovative Memory Devices: II. Molecular Adsorption on Electrodes and the Asymmetric Response

“…This value is quite similar to 6.124 D we calculated at the same level of calculation in gas phase, and to 6.245 D we calculated exactly from the DFT/B3LYP/N07D electron density of 8HQ(II) in gas phase. 8 The dipole moment is oriented 15.0°from the normal to the Al surface, and it is directed toward the vacuum region.…”

“…These are the Gibbs free energies required to transfer an electron from the aluminum electrode to an 8HQ molecule and vice versa: ϕ e = ϕ Al − EA ϕ h = IP − ϕ Al where ϕ Al is the aluminum work function, and EA and IP are the 8HQ electron affinity and ionization potential, respectively. In the first part of this work, the average experimental value of the bare aluminum work function has been considered, and we calculated EA and IP of 8HQ in vacuum and solvated in amorphous polystyrene within the polarizable continuum model (see Table 6 in ref ). At that level we neglected possible phenomena at the aluminum interface, such as molecular adsorption, or partial solvation of 8HQ molecules at boundaries, which could affect the injection barriers at zero bias, and the electric response under low voltage applications.…”

“…A deeper understanding of the key physical/chemical factors tuning the behavior of organic electronic devices is a mandatory prerequisite for optimizing the structure and the composition of the materials, for approaching the performances, the stability, and the reliability of the corresponding semiconductor devices, or even for new components . The present study represents the second step of a project aimed at understanding the various aspects of the Yang’s device, using the tools of computational chemistry. Our first aim is to describe the low-conductivity regime of the device, throwing light on the thermodynamics to be obeyed for electronic switching to the high-conductivity state.…”

“…In the first part of this study, 8 the calculations of the structural and electronic properties of the PS-8HQ mixture uncovered the role that PS-solvation plays in lowering both the hole and the electron injection barriers, determining the voltage to be applied for the conductivity switching. The latter aim was achieved by calculating the electron affinities (EA) and ioniza-environment and by considering the experimental value of the aluminum work function.…”