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

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 electric-field 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.

本研究涉及的50纳米厚度的聚苯乙烯（PS）薄膜，应用于若干创新型存储设备，其中含有8-羟基喹啉（8HQ）分子和金纳米粒子。本工作中，我们测试了一个模型，其中分子局域性质直接反映在复杂系统的宏观行为上。该研究聚焦于在连续方案中模拟的8HQ-PS混合物的结构和电子性质：一个采用极化连续介质模型（PCM）的8HQ分子，其可靠性已通过与8HQ-PS晶体结构的周期性DFT计算的对比得到验证。采用B3LYP、LC-PBE和M052X泛函以及极化双ζ基组，在DFT水平上对8HQ的酮-烯醇互变异构进行了全面研究。通过在CCSD(T)水平上使用aug-cc-pVDZ基组进行单点计算，对所获得的结构（极小值和过渡态）的能学进行了精细调整。我们的计算预测，对于孤立和PS溶剂化的中性8HQ，烯醇互变异构是最稳定的，其互变异构能垒远大于工作条件下的热能。对于带电（正负）8HQ，则呈现出相反的趋势，酮互变异构是最稳定的。在第一次与铝电极的弱相互作用近似中，计算8HQ分子的电子亲和力和电离势时也考虑了电场效应。从这些结果中产生的电子和空穴注入势垒与实验观察结果相吻合。