Synthesis of heterometallic group 13 nanoclusters and inks for oxide thin-film transistors.

We have recently reported high-yielding syntheses of two inorganic Group 13 metal-hydroxide nanoclusters: [Ga13(m3OH)6(m-OH)18(H2O)24(NO3)15] (“flat” Ga13, 1, Figure 1) and [Al13(m3-OH)6(m-OH)18(H2O)24(NO3)15] (“flat” Al13). [1, 2] The {M(m3-OH)6M6(m-OH)6} central fragment of these clusters forms a planar core with six additional M(H2O)4 groups bound to the core by two m-OH bridges. The outer metal ions alternate above and below the plane formed by the central seven metal ions. Prior synthetic preparation of Group 13 metal-hydroxide compounds such as these has proven difficult. Their syntheses often require caustic or acidic conditions and elevated temperatures and pressures to provide clusters, often in low yields. Crystallization periods of months or even years are typical. Owing to these difficulties, relatively few discrete Group 13 metal-hydroxide clusters have been synthesized, though several striking examples of aluminum 4, 6,7] and gallium 5, 8, 9] complexes have been reported. Both hydrated clusters and those stabilized by organic ligands are known, with a larger variety of ligand-supported clusters having been isolated, owing to enhanced stability resulting from lower charge density. In the case of these inorganic and ligand-supported compounds, neither heterometallic nor indium-containing clusters are known. However, in the case of Keggin-Al13 clusters, the central tetrahedral metal can be substituted, forming M1Al12 structures (M = Al, Ga, or Ge, with others suggested). To our knowledge, no heterometallic Group 13 metal-hydroxide clusters with multiple substitution have been reported. Furthermore, the lowyielding, challenging syntheses often associated with these clusters have prevented attempts at exploring applications requiring large quantities of such compounds. In order to address synthetic difficulties and to explore the use of these clusters as precursors for materials, we have developed an improved synthesis of heterometallic Group 13 nanoclusters. There has been recent interest in the use of nanoscale cluster precursors to synthesize new materials. The difficult syntheses of Group 13 metal-hydroxide clusters have mostly prevented their use in these applications. Most solution precursors for printed oxide films involve controlled hydrolysis of metal–organic compounds and the condensation of metal-hydroxo sols that are then pyrolyzed to form the oxide. Such films are beset by a variety of density, defect, and segregation issues relating to the inhomogeneous nature of the sol, retention of significant organic components, or oxygen nonstoichiometry associated with organic burnout. From this perspective, soluble all-inorganic, heterometallic hydroxo clusters, such as the “flat” M13 system, provide model precursors and an entirely inorganic, rapid, low-volume-loss condensation pathway, eliminating the aforementioned detrimental effects of organic moieties. Herein, we present a new heterometallic gallium–indium cluster, [Ga7In6(m3-OH)6(m-OH)18(H2O)24(NO3)15] (“flat” Ga7In6, 2, Figure 1). This compound can be synthesized reliably, in yields ranging from 25% to 95 %, by utilizing two different nitroso additives. 2] Clusters 1 and 2 can both be prepared in gram-scale quantities, which enables the unprecedented use of these nanoclusters as single-source solution precursors for the deposition of oxide-semiconductor thin Figure 1. Representations of the molecular structures of [Ga13(m3-OH)6(m-OH)18(H2O)24(NO3)15] (1, left) and [Ga7In6(m3-OH)6(m-OH)18(H2O)24(NO3)15] (2, right), determined by single-crystal X-ray diffraction. Ga and In are shown as dark and light spheres, respectively; hydroxo and aquo ligands are shown as stick representations.