Metal-Seeded Growth Mechanism of ZnO Nanowires

Abstract

The widely applied metal-catalyzed growth mechanism of ZnO nanowires (NWs) is investigated by advanced methods of transmission electron microscopy and is discussed with respect to thermodynamic growth conditions. Au catalyst particles do not contain a substantial amount of Zn proving a solid Au catalyst at 1173 K growth temperature. This result is owed to the high equilibrium Zn partial pressure over Au–Zn alloys which in turn leads to a very low sticking coefficient of Zn from vapor and prevents alloying. Growth rates of ZnO NWs were measured between 5.5 nm s–1 and 36 nm s–1 as a function of oxygen partial pressure. The enhanced growth rate at higher oxygen partial pressures is explained by an increased sticking coefficient of Zn atoms at the Au catalyst. A growth mechanism is proposed which is quite different from the classic vapor–liquid–solid (VLS) mechanism: Zn alloys only in a thin surface layer at the catalyst and diffuses to the vapor–catalyst–NW triple phase line. There, together with oxygen, ZnO ledges nucleate which grow laterally to inner regions of the ZnO–Au heterointerface where Zn and oxygen can diffuse and finally promote NW growth in a rather kinetically controlled process. The geometry of the ZnO–Au interface — planar or stepped — and the associated diffusional transport properties are shown to be determined by the orientation relationship between Au and ZnO and hence by the atomic structure of the interface.