Chemistry Reference
In-Depth Information
Bare gold cluster cations, Au
n
+
(
n
12-72), generated via laser vaporisation
using a Smalley-type source were mass selected and isolated in a Penning trap
[
198
]. The mass-selected clusters were then bombarded with an electron beam
(typically ca. 150 eV for 1.2 s) generated from a rhenium filament. Electron impact
resulting in the dissociation and further ionisation of mass-selected gold clusters
reveals that small gold clusters, Au
n
z
+
(
n
¼
30) where
z
¼
1 or 2, exhibit an odd-
even alternation in their abundance with preference to even electron clusters. For
Au
n
+
,
n
is generally an odd number and for Au
n
2+
n
is generally an even number.
3.2
Ion Mobility
The mobility of gas-phase ions through a gas and under the influence of an electric
field is a useful property to exploit in order to develop techniques to separate
nanoparticles and nanoclusters based on their size, shape and charge [
204
]. When
coupled to mass spectrometry, termed IMS [
205
], it allows for accurate measure-
ments of the particle mass and therefore its composition. The marriage of this
technique with theoretical calculations has proven to be very powerful in determin-
ing the structures of gas-phase cluster ions [
206
]. The use of IMS is gaining grounds
in the analysis of clusters. Weis has reviewed the use of IMS in fundamental studies
of metal clusters, including gold clusters [
207
], and a chapter in this topic, by
A. Fielicke et al., highlights work done on bare AuNC ions. Here we briefly review
IMS studies on bare AuNCs (Sect.
3.2.1
) and ligated AuNCs (Sect.
3.2.2
).
3.2.1 Bare Gold Nanoclusters
In early work, laser ablation coupled with IMS and density functional theory
calculations was used to determine the structures of bare gold anions and cations,
Au
n
and Au
n
+
(
n
up to 13) [
208
,
209
]. Cationic clusters were found to have planar
structures for
n
3-7 and form 3D structures at
n
8. Anionic clusters on the
other hand are planar for
n
3-11, for
n
¼
12 both a planar and a three-
dimensional structure seem to coexist, and for
n
13, only a three-dimensional
structure is observed (Fig.
19
). Kappes also studied mixed silver-gold clusters
Ag
m
Au
n
+
(
m
+
n
)
¼
6. Triangular, rhombus (or Y-shaped) and connected triangles
(or trigonal bipyramid) were observed as the main structures for trimers, tetramers
and pentamers, respectively.
<
3.2.2 Ligated Gold Nanoclusters
Ion mobility was also used to study ligated gold clusters. Lenggoro et al. [
210
] have
reported the use of ES-DMA (electrospray-differential mobility analysis) to
study AuNPs. This technique allows the segregation of the nanoclusters by size
