PNNL

Abstract

Materials with tunable properties are an essential part of solving the challenges related to integrated computer memories. Metal oxygen nanoclusters such as polyoxometalates (POMs) have emerged as interesting candidates for such applications, due to their unique switching properties. Although the electronic and switching properties of POMs can be greatly influenced by modifying them with organic substituents, this effect has so far been largely unexplored. In this work, we assessed the importance of electron-withdrawing and electron-donating ligand substituents on the material properties of POMs as potential resistive random-access memory (ReRAM) components by studying their behavior in different states of matter. Our study combines cyclic voltammetry, negative ion photoelectron spectroscopy, analytical and preparative mass spectrometry, as well as scanning tunneling microscopy and spectroscopic characterization of a series of hybrid Lindqvist-type hexavanadates TBA2[V6O13((OCH2)3CCH2OH)2] (TBA2V6-OH), TBA2[V6O13((OCH2)3CMe)2] (TBA2V6-Me), TBA2[V6O13((OCH2)3CNHCOCH2Cl)2] (TBA2V6-Cl), and TBA2[V6O13((OCH2)3CNHCOCH2-OOCC10H15)2] (TBA2V6-Ad). The hybrid POMs show different redox characteristics in solution due to different electronic effects of their peripheral ligand substituents, consistent with their intrinsic redox properties that were investigated experimentally in the gas phase as well as with computational methods. Scanning tunneling microscopy (STM) investigations of the hexavanadate POMs deposited on Au(111) surfaces using drop casting and ion soft-landing revealed different lateral electron density distributions depending on the used deposition method. Compared to their redox behavior in solution, changing the ligand substituents on surfaces resulted in no significant effect on the potential and, thus, no effect on the resistance steps in the current-voltage profiles. However, while the current-voltage characteristics do not change, the peripheral metal-free substituents in the trisalkoxide framework of Lindqvist-type hexavanadate molecules influence the adsorption and switching stability of these POMs on gold. This work demonstrates the importance of ligand’s electronic factors on the intrinsic stability of the hybrid POMs and highlights the noticeable differences between hexavanadate’s redox properties in solution (which follow the trend observed in the gas phase) and hexavanadate’s redox-dependent switching properties on conducting surfaces. Importantly, their multi-state switching behavior is not significantly altered by the different type of substituent at the periphery of the trisalkoxo ligands.