Atomic Structure of a Spinel-Like Transition ${\mathrm{Al}}_2{\mathrm{O}}_3(100)$ Surface

We study a crystalline epitaxial alumina thin film with the characteristics of a spinel-type transition ${\mathrm{Al}}_2{\mathrm{O}}_3(100)$ surface by using atom-resolved noncontact atomic force microscopy and density functional theory. It is shown that the films are terminated by an Al-O layer rich in Al vacancies, exhibiting a strong preference for surface hydroxyl group formation in two configurations. The transition alumina films are crystalline and perfectly stable in ambient atmospheres, a quality which is expected to open the door to new fundamental studies of the surfaces of transition aluminas.

Figure 1

Top view ball models of the (a) ${\mathrm{MgAl}}_2{\mathrm{O}}_4(100)$ and (b) $s\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(100)$ surfaces. Surface unit cells, high symmetry directions, and Al vacancies are marked with dashed lines.

Figure 2

NC-AFM images of (a) a ${\mathrm{MgAl}}_2{\mathrm{O}}_4(100)$ surface and (b)–(d) $\mathrm{Al}/{\mathrm{MgAl}}_2{\mathrm{O}}_4(100)$ surfaces with increasing Al coverage.

Figure 3

(a) XPS spectra of the Al $2p$ region. (b) Normal incidence LEED pattern of $s\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(100)$.

Figure 4

Partially atom-resolved NC-AFM image of the $s\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(100)$ recorded in constant detuning mode with $d{f}_{\text{set}}=-56\mathrm{Hz}$, $U_{\text{bias}}=-0.4\mathrm{V}$, and $A_{p\text{−}p}=8\mathrm{nm}$. (a) Depressions are marked with dashed white circles, and the inset shows a line scan across one of these depressions. (b) Square lattice grid superimposed on the NC-AFM image.

Figure 5

Interface energy plot of the most relevant $s\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ films with respect to the bulk ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ structure. Film I, all vacancies are replaced with Al; film II, vacancies like bulk $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$; films III–V, including adsorbed H and OH groups.