On-Surface Single-Molecule Identification of Mass-Selected Cyclodextrin-Supported Polyoxovanadates for Multistate Resistive-Switching Memory Applications

Abstract

Polyoxovanadate anions immobilized on conducting substrates are promising candidates for molecule-based memory technologies. However, strong electronic interactions with the substrate and the formation of larger agglomerates remain bottlenecks on the way to their technical application. We demonstrate that ion soft-landing of mass selected cyclodextrin-functionalized hexavanadates enables deposition of these host–guest complexes as single memory units on surfaces without their quaternary ammonium countercations from solution. Cyclodextrin provides a stabilizing “shell” for the dianions. Multistate switching evidenced by the change in molecular resistance has been detected using scanning tunneling microscopy and spectroscopy and attributed to reduction of individual vanadium (+V) centers.

Experimental observation of the role of countercations in modulating the electrical conductance of Preyssler-type polyoxometalate nanodevices

“…In this experiment, the size of the electron cloud density measured by STM largely extends the geometric size of the POM due to the hybridization of the POM and the counterion with the metal surface, with a stronger hybridization with a TBA + cation rather than the POM encapsulated in a cyclodextrin cage (cation free). 25 This hybridization is also the key factor for fixing the molecule/electrode energy coupling in the SEL model (eqn (1)). Thus, we hypoth-Table 2 SEL parameters ε 0 , Γ 1 and Γ 2 fitted on the mean I ¯-V (Fig.…”

“…23,24 For example, for a single POM on a metallic surface, the electron cloud density around the POM, as observed with a scanning tunneling microscope (STM), depends on the counterions that modify the hybridization between the POM and the metal surface. 25 A recent theoretical study shows that the presence of a counterion (whatever its nature, here tetramethylammonium (TMA) vs. Cs + ) moves the LUMO (lowest unoccupied molecular orbital) of W 18 O 54 (SO 3 ) 2− based MJs closer to the Fermi energy of the electrodes (∼0.08 eV above the Fermi energy in both the cases). 26 Despite this similar position of the LUMO, the computed current-voltage (I-V) curves showed slight differences in the shape (more asymmetric I-V with Cs + ).…”

Large anion binding in water

“…With large superchaotropic anions a new high-affinity binding motif or anchoring unit for CDs has been identified, which opens several lines of applications. 41,43,49,96,155,156 For example, the dodecaborate anion B 12 H 11 SH 2− , as an auxiliary anchor residue, has been tethered to an organic moiety to synthesize water-soluble anchor dyes with high affinity to CDs. 43 Upon complexation with CDs, dodecaborate-substituted dyes showed marked changes in their UV-visible and fluorescence properties, which enabled their application for indicator displacement assays.…”

Increasing the redox switching capacity of Lindqvist-type hexavanadates by organogold post-functionalisation

“…Herein, we demonstrate that the covalent functionalisation of trisalkoxo-supported Lindqvist-type hexavanadates by gold(I) complexes 25 bearing phosphine or carbene ligands can be used as an efficient lever to modulate the molecular multi-state switching in scanning tunnelling microscopy (STM) studies of vanadium-containing POMs 26 at room temperature and low potentials. This is an important finding, especially with regard to the ability of trisalkoxo-supported Lindqvist-type hexavanadates [27][28][29] to switch from high-resistive state (V IV : V V ratio = 0 : 6) to low-resistive state (V IV : V V ratio = 4 : 2) at room temperature and applied potentials below 2 volts.…”

“…This discrepancy is related to hybridisation effects of the POM anions on the metal surface 27 and the counter cations in their immediate vicinity. 29 It is noteworthy that the observed particle distribution, their STM-observed sizes, and even their stability on the surface upon subsequent STM scans (Fig. S15, ESI †) depend on the structure of the organogold moiety as well as on the solvent used for deposition.…”