This Perspective article highlights some of the traditional and non-traditional analytical tools that are presently used to characterize aqueous inorganic nanoscale clusters and polyoxometalate ions. The techniques discussed in this article include nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), dynamic and phase analysis light scattering (DLS and PALS), Raman spectroscopy, and quantum mechanical computations (QMC). For each method we briefly describe how it functions and illustrate how these techniques are used to study cluster species in the solid state and in solution through several representative case studies. In addition to highlighting the utility of these techniques, we also discuss limitations of each approach and measures that can be applied to circumvent such limits as it pertains to aqueous inorganic cluster characterization.

Raman spectroscopy, infrared spectroscopy, and quantum mechanical computations were used to characterize and assign observed spectral features, highlight structural characteristics, and investigate the bonding environments of [M13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 (M = Al or Ga) nanoscale clusters in the solid phase and aqueous solution. Solid-phase Raman spectroscopy was used to reveal that the metal-oxygen (M-O) symmetric stretch (breathing mode) for the Al13 cluster is observed at 478 cm(-1), whereas this same mode is seen at 464 cm(-1) in the Ga13 cluster. The hydroxide bridges in each cluster are weakly Raman active but show slightly stronger infrared activity. The breathing modes associated with the clusters in the solid state are not clearly visible in aqueous solution. This change in behavior in the solution phase may indicate a symmetry breaking of the cluster or exchange events between protons on the ligands and the protic solvent. Overall, each cluster has several unique vibrational modes in the low wavenumber region (<1500 cm(-1)) that are distinct from the parent nitrate salt and other polymeric species with similar structure, which allows for unambiguous identification of the cluster in solution and solid phases.

[AlxIny(μ3-OH)6(μ-OH)18(H2O)24](NO3)15 hydroxy-aquo clusters (AlxIn13-x) are synthesized through the evaporation of stoichiometrically varied solutions of Al13 and In(NO3)3 using a transmetalation reaction. Several spectroscopic techniques ((1)H NMR, (1)H-diffusion ordered spectroscopy, dynamic light scattering, and Raman) are used to compare AlxIn13-x to its Al13 counterpart. A thin film of aluminum indium oxide was prepared from an Al7In6 cluster ink, showing its utility as a precursor for materials.

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Electrochemical reduction is used to synthesize indium-gallium-hydroxide-nitrate nanoclusters which are shown to be promising precursors for thin-film transistors.

Thin films formed by the condensation of metal oxo–hydroxo clusters offer a promising approach to ultrahigh-resolution patterning including next-generation photolithography using extreme ultraviolet (EUV) radiation and electron-beam lithography. In this work, we elucidate the thermal and radiative mechanisms that drive the chemical transformations in these materials and therefore control the patterning performance. Beginning from aqueous hafnium clusters, peroxide and sulfate additions serve to modify the clusters and, upon spin coating to form a thin film, provide the chemical contrast necessary to create resist. The coordination and functionality of peroxide and sulfate in hafnium-based metal oxo–hydroxo clusters were monitored at various stages of the patterning process which provided insight into the chemical and structural evolution of the material. Peroxide serves as the radiation sensitive species while sulfate enhances solubility and controls the concentration of hydroxide in the films. Peroxide and hydroxide species decompose via radiative and thermal energy, respectively, to form hafnium oxide; controlling these processes is central to the function of the resist.

We describe a process to produce aqueous precursor solutions of the flat -Al 13 hydroxo cluster (Al13(μ3-OH)6(μ2-OH)18(H2O)24(NO3)15) via stoichiometric dissolution of bulk Al­(OH)3(s) in HNO3(aq). We highlight its facility by demonstrating high yields and large-scale synthesis. X-ray diffraction confirms formation of a single-phase product, and Raman spectra show characteristic O-Al-O vibrational modes, both techniques confirming the identity of the flat -Al 13 cluster in the bulk. 27Al NMR spectroscopy and dynamic light scattering also confirm the presence of the cluster in aqueous solution. We show the as-prepared solution produces smooth and continuous thin films via spin-coating. In capacitors, the films exhibit low leakage currents (near 10 nA/cm2) and dielectric constants expected for amorphous Al2O3. Because the precursor preparation requires no postsynthesis purification, it is readily scalable to large volumes.

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