Insights from Adsorption and Electron Modification Studies of Polyoxometalates on Surfaces for Molecular Memory Applications

This Account highlights recent experimental and theoretical work focusing on the development of polyoxometalates (POMs) as possible active switching units in what may be called "molecule-based memory cells". Herein, we critically discuss how multiply charged vanadium-containing POMs, which exhibit stable metal-oxo bonds and are characterized by the excellent ability to change their redox states without significant structural distortions of the central polyoxoanion core, can be immobilized best and how they may work optimally at appropriate surfaces. Furthermore, we critically discuss important issues and challenges on the long way toward POM-based nanoelectronics. This Account is divided into four sections shedding light on POM interplay in solution and on surfaces, ion soft-landing of mass-selected POMs on surfaces, electronic modification of POMs on surfaces, and computational modeling of POMs on surfaces. The sections showcase the complex nature of far-reaching POM interactions with the chemical surroundings in solution and the properties of POMs in the macroscopic environment of electrode surfaces. Section 2 describes complex relationships of POMs with their counter-cations, solvent molecules, and water impurities, which have been shown to exhibit a direct impact on the resulting surface morphology, where a concentration-dependent formation of micellar structures can be potentially observed. Section 3 gives insights into the ion soft-landing deposition of mass-selected POMs on electrode surfaces, which emerges as an appealing method because the simultaneous deposition of agglomeration-stimulating counter-cations can be avoided. Section 4 provides details of electronic properties of POMs and their modification by external electronic stimuli toward the development of multiple-state resistive (memristive) switches. Section 5 sheds light on issues of the determination of the electronic structure properties of POMs across their interfaces, which is difficult to address by experiment. The studies summarized in these four sections have employed various X-ray-scattering, microscopy, spectroscopy, and computational techniques for imaging of POM interfaces in solution and on surfaces to determine the adsorption type, agglomeration tendency, distribution, and oxidation state of deposited molecules. The presented research findings and conceptual ideas may assist experimentalists and theoreticians to advance the exploration of POM electrical conductivity as a function of metal redox and spin states and to pave the way for a realization of ("brain-inspired") POM-based memory devices, memristive POM-surface device engineering, and energy efficient nonvolatile data storage and processing technologies.

Computational study of the staircase molecular conductivity of polyoxovanadates adsorbed on Au(111)

This computational study presents the molecular conduction properties of polyoxovanadates V₆O₁₉ (Lindqvist-type) and V₁₈O₄₂, as possible successors of the materials currently in use in complementary metal–oxide semiconductor (CMOS) technology.

A universal strategy for fabrication and morphology control of polyoxometalate-based metal–organic frameworks

Fabrication of POM@MOFs by utilizing the oxidization of POMs to metals and the effect of polyoxoanion charge on POM@MOF morphology.

Controlled chiral arrangement of silver nanoparticles in supramolecular gels modulated by the cooling rate

Opposite helical arrangements of silver nanoparticles can be in situ achieved in organogels from a single gelator at different cooling rates.

Structure, stability, and electronic interactions of polyoxometalates on functionalized graphene sheets

Polyoxometalates (H(3)PMo(12)O(40), H(3)PW(12)O(40), H(4)PMo(11)VO(40)) supported on oxygen- and alkyl-functionalized graphene sheets were investigated. Discrete molecular species were directly observed by electron microscopy at loadings below 20 wt.%. The interaction between the polyoxometalates and the graphene surface was found to significantly impact their vibrational spectra and a linear correlation between the frequency of the M-O(c)-M vibration and the dispersion was evidenced by FTIR. While bulk-like electronic properties were observed for small aggregates (2-5 nm), UV-vis spectroscopy and cyclic voltammetry revealed changes in the electronic structure of isolated molecular species as a result of their interaction with graphene. Because of the ability to disperse alkyl-functionalized graphene in a variety of polar and nonpolar solvents, the materials synthesized in this work provide an opportunity to disperse polyoxometalates in media in which they would not dissolve if unsupported.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Polyoxometalate-decorated nanoparticles

Polyoxometalate cluster anions (POMs) control formation and morphology, and serve as protecting ligands, for structurally and compositionally diverse nanostructures. While numerous examples of POM-protected metal(0) nanoparticle syntheses and reactions can now be found in the literature, the use of POMs to prepare nano-scale analogs of binary inorganic materials, such as metal-oxides, sulfides and halides, is a relatively recent development. The first part of this critical review summarizes the use of POMs as protecting ligands for metal(0) nanoparticles, as well as their use as templates for the preparation of new inorganic materials. Here, key findings that reveal general trends are given additional emphasis. In the second part of the review, new information concerning the structure of POM-protected metal(0) nanoparticles is systematically developed. This information, obtained by the combined use of cryogenic transmission microscopy (cryo-TEM) and UV-vis spectroscopy, provides a new perspective on the formation and structure of POM-decorated nanoparticles, and on the rational design of catalytic and other functional POM-based nano-assemblies.

Role of the alkali-metal cation size in the self-assembly of polyoxometalate-monolayer shells on gold nanoparticles

Polyoxometalate (POM)-monolayer stability constants, K, for three POM anions vary with the cation size, in the same order as that for increasing ion-pair formation with α-SiW(11)O(39)(8-) (1) in the early nucleation phase of monolayer self-assembly: Li(+) < Na(+) < K(+) < Cs(+). Cryo-TEM images demonstrating the use of the cation size to rationally control monolayer formation provide definitive evidence that the POM monolayers are electrostatically stabilized (ionic) shells, analogous in that respect to the monolayer walls of "hollow" POM-macroanion vesicles.