Chemistry Reference
In-Depth Information
Fig. 3 Sketch of the transition from a metallic bulk state (a) via a large cluster (b) to a small
molecule (c) indicating the changes of the electronic situations
sharp energy levels when a molecular cluster
1
is reached (c). What are the experi-
mental results?
The most relevant physical method for investigating individual metal clusters
and nanoparticles electronically is to determine the current (I) voltage
(U) characteristic using a scanning tunneling microscopy (STM) tip to contact
an individual particle on a conductive surface via the obligatory ligand shell. With
this technique, single-electron transitions (SET) between tip, particle, and sub-
strate can be observed, if some conditions are fulfilled. The nonconducting
protecting ligand shell plays an important role, since neither the STM tip nor the
conductive substrate touches the particle itself. SET can only be observed if
the electrostatic energy
E
C
¼
e
2
/2
C
(
C
¼
capacity) is very large compared to
the thermal energy
E
T
¼
k
B
T
(
k
B
¼
Boltzmann's constant). Since
C
¼ εε
0
A
/
d
(
ε ¼
dielectric constant,
ε
0
¼
electric field constant,
d
¼
distance of electrodes
from metal core,
A
¼
surface area), the thickness of the ligand shell is important
for the value of
C
. At very low temperatures single-electron transitions can be
observed using rather large nanoparticles. For instance, the I-U characteristic of a
15 nm Pd particle, protected by a shell of H
2
NC
6
H
4
SO
3
Na molecules and
investigated at 295 K, clearly shows bulk properties, i.e., a linear I-U relationship
following Ohm's law. However, at 4.2 K a so-called Coulomb blockade (CB) occurs,
indicated by an interruption of conductivity between
55 and +55 mV by a single
electron [
13
]. Figure
4
shows the I-U characteristics at both temperatures.
1
The difference between the expression “cluster” and “nanoparticle” is not sharp. In the following
we will use “cluster” for particles of a discrete number of atoms, whereas “nanoparticle” is used
for less exactly defined species, allowing a certain size distribution.
