Synthesis and evaluation of κ²-β-diketonate and β-ketoesterate tungsten(vi) oxo-alkoxide complexes as precursors for chemical vapor deposition of WO_x thin films

Abstract

Reactions of [WO(OR)4]x (x = 1, 2) complexes with bidentate ligands (LH = acacH, tbacH, dpmH, tbpaH) afforded complexes : [WO(OCH3)3(acac) (); WO(OCH2CH3)3(acac) (); WO(OCH(CH3)2)3(acac) (); WO(OCH3)3(tbac) (); WO(OCH2CH3)3(tbac) (); WO(OCH(CH3)2)3(tbac) (); WO(OCH2CH3)3(dpm) (); WO(OCH(CH3)2)3(dpm) (); WO(OCH2C(CH3)3)3(acac) (); WO(OCH2C(CH3)3)3(tbac) (); WO(OCH2C(CH3)3)3(dpm) (); WO(OCH2C(CH3)3)3(tbpa) (); WO(OC(CH3)3)3(tbac) ()]. The synthesis is facilitated by the lability of the bridging ligands of the [WO(OR)4]2 complexes in solution, which provides a pathway for exchange of L with an alkoxide ligand. Thermogravimetric analysis and the conditions for sublimation or distillation of demonstrate that they have sufficient vapor pressure and thermal stability for volatilization in a conventional Chemical Vapor Deposition (CVD) reactor. High solubility in hydrocarbon and ether solvents establishes that the complexes are also potential candidates for Aerosol-Assisted Chemical Vapor Deposition (AACVD). AACVD from on ITO or bare glass resulted in growth of continuous, dense and amorphous thin films of substoichiometric WOx between 250-350 °C and nanorods of W18O49 above 350 °C.

Synthesis of tungsten oxo fluoroalkoxide complexes WO(OR)3L as precursors for growth of WOx nanomaterials by aerosol-assisted chemical vapor deposition

“…For β-ketoesterate complexes 4 and 8, the base peak is the loss of the chelating ligand while for the β-diketonate complexes 2 and 5, the base peak is loss of an alkoxide. This greater lability of the β-ketoesterate ligands is consistent with structural data on similar tungsten complexes in which β-diketonate ligands have shorter bonds (and thus stronger bonding) to tungsten than β-ketoesterates [38]. There is also little or no [M-OR] + peak for the β-ketoesterates 4 and 8.…”

“…Loss of HL and A f from 1, 4, and 8 suggests that all three complexes share features of their decomposition mechanisms. These fragments are consistent with a pathway in which a hydrogen on an alkoxide undergoes nucleophilic attack by a neighboring β-diketonate/β-ketoesterate (Scheme 2), a step invoked in decomposition of related metal alkoxide complexes[38,55]. The additional fragments observed for 4 and 8 suggest a decomposition pathway enabled by the tert-butyl acetoacetate ligand.…”

“…The bidentate β-diketonate or ketoesterate ligand was chosen to increase stability of the complex to air and moisture through additional occupancy of the coordination sphere. We have previously compared β-ketoesterate ligands to β-diketonates in related series of precursors of the type WO(OR) 3 L (R = Me, Et, i Pr, t Bu and Np, L = acac, dpm and tbac) and determined that the β-ketoesterates offer the advantage of facile fragmentation pathways involving the ester group, [38] a conclusion also reached in other studies [47].…”

“…However, these complexes are air-and water-sensitive, which complicates their practical use. Modification of these complexes with chelating ligands has been reported to improve their stability and a series of these WO(OR) 3 L complexes has been demonstrated to serve as viable precursors for AACVD of WO x films and nanostructures [9,36,38]. Fluorination of ligands has been shown to increase the volatility of complexes as a result of decreased intermolecular interactions due to the repulsion produced by the fluorine atoms [39,40].…”

Aerosols: A Sustainable Route to Functional Materials

“…We have been able to demonstrate this for the deposition of ZnO, In 2 O 3 , and VO 2 thin films. β‐Diketonate ligands have also been used to deposit WO 3 and CuO . There are also a wide variety of β‐diketonate ligands known for metals including: Ti, Hf, Ta, Sr, Ca, Mg, and Ba, therefore a wide range of metal oxide materials are accessible via such precursors.…”