Chemistry Reference
In-Depth Information
Figure 5.11. TMAFM image of a thin film (thickness
∼
1
µ
m) of
α
-
p
-NPNN on
×
NaCl(100): 1
m. Reprinted from
Surface Science
, Vol. 415, J. Fraxedas,
J. Caro, A. Figueras, P. Gorostiza and F. Sanz,
Dislocation hollowcores observed on
surfaces of molecular organic thin films: p-nitrophenyl nitronyl nitroxide radical
,
241-250, Copyright (1998), with permission from Elsevier.
µ
m
1
µ
5.5 Nanometre-scale surface morphology
The nanometre-scale morphology of the surfaces of the films can provide valuable
information related to the growth mechanisms. Figure 5.11 shows a TMAFM image,
measured at ambient conditions, of a 1
µ
m thin film of
p
-NPNN grown by PVD
on an
ex situ
cleaved NaCl(100) substrate. Such
p
-NPNN/NaCl(100) films are also
highly oriented, as when grown on glass substrates, with the molecular
ab
-planes
parallel to the substrate's (100) planes (Caro
et al.
, 1998). The image reveals a
random distribution of dislocations. These dislocations are spirals associated with
screw dislocations coupled in pairs of opposite sign, each spiral emerging from a
hollow core.
Hollow cores (black filled circles in the image) are empty tubes located at the
dislocation core and they originate from the stress field generated during growth.
F. C. Frank predicted the formation of such hollow cores along dislocation lines
with Burger's vectors
Λ
with moduli larger than 1 nm (Frank, 1951). As you will
soon see, he was right, for the height of the steps is 1.2 nm, the distance between two
