posted on 2013-02-06, 00:00authored byHeike Simon, Tobias Krekeler, Gunnar Schaan, Werner Mader
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.