Thin films: Glassy alumina

Nuclear magnetic resonance reveals the structure of amorphous aluminum oxide.

The atomic structure of a glassy form of aluminum oxide (Al₂O₃) has been revealed for the first time.

Thin layers of Al₂O₃ are widely used as protective films, as supports for catalysts and in microelectronic devices. While some thin films of Al₂O₃are crystalline, with regular arrangements of atoms, those formed from a vapor are amorphous solids (glasses) that do not have a repeating atomic structure, giving them different structural and chemical properties. The structure of pure Al₂O₃ glass, however, has not been studied before.

Fig. 1: Schematic structure of amorphous aluminium oxide (top) composed mainly of four- and five-coordinated aluminum atoms (left), and crystalline α-Al₂O₃ (bottom) containing six-coordinated aluminum atoms.

Sung Keun Lee of Seoul National University and colleagues¹ have now used a two-dimensional (2D) nuclear magnetic resonance (NMR) technique to investigate the structure of amorphous Al₂O₃. Their method, 2D triple quantum magic angle spinning NMR, revealed that aluminum atoms can adopt three different atomic arrangements in a sample of amorphous Al₂O₃ just 1.4 micrometers thick.

In crystalline α-Al₂O₃, each aluminum atom is bonded to six oxygen atoms. But in its glassy form, about 55% of the aluminum atoms are four-coordinated (bonding to four oxygen atoms), and 42% are five-coordinated (see image).

As 2D NMR methods generally require relatively large amounts of sample, they have not previously been used on thin films in this way. “This is the first application of 2D NMR on any thin film material,” says Lee. “Our methods and results have the potential for revolutionizing the structural study of thin films and the surfaces of amorphous materials.”

The team also found that after the sample had been heated to 800 °C, the five-coordinated aluminum atoms transformed into four- and six-coordinated centers, indicating that the sample had begun to crystallize. After heating to 1200 °C, only six-coordinated aluminum atoms remained, forming crystalline α-Al₂O₃.

Lee explains that since five-coordinated aluminum may have stronger catalytic activity than four- and six-coordinated aluminum, it is possible that amorphous Al₂O₃ could be a more active catalyst than its crystalline form. The large number of five-coordinated aluminum atoms also suggests that there are many three-coordinated oxygen atoms in the structure, adds Lee, contradicting the traditional view that oxygen atoms should bond to just two other atoms.