Chemistry Reference
In-Depth Information
Fig. 4 Simultaneously acquired STEM-HAADF (a) and STEM-BF (b) images taken from the
edge of an Au nanorod. Reproduced from [
49
]
is a time-consuming process, this study showed that it can be used to build a 3D
representation of interfacial structure in individual bimetallic nanoparticles.
4.2 Spectroscopy
Energy-dispersive X-ray spectroscopy (EDX) is a form of spectroscopy that utilises
the detection of X-ray emissions that occur when beam electrons interact with the
sample. These X-ray emissions are entirely elementally specific. Given sufficient
X-ray counts, statistical methods can be used to calculate accurately the relative
quantities of elements present in the sample from EDX spectra [
53
]. The technique
of elemental mapping is particularly useful for analysing bimetallic samples and
has been used extensively in this regard.
In contrast to EDX, electron energy loss spectroscopy (EELS) tends to have a
better signal to noise ratio, thus improving spatial resolution. EEL spectra show the
energy lost by individual beam electrons as they are transmitted through the sample.
As the electronic interactions are derived from the dielectric response, they are
elementally specific [
54
]. Thus EELS can be an effective tool to measure the
elemental composition of bimetallic samples, particularly when used in combina-
tion with elementally sensitive STEM-HAADF imaging. The better signal to noise
ratio of EELS compared to EDX results in improved spatial resolution. However, in
comparison to EDX, EELS spectra can be more complex to interpret, as the
position and appearance of spectral features can be affected by the electronic
structure of the sample. EELS is also limited to signals that are within the range
of the spectrometer and so is of less use in measuring high energy loss signals (over
1,000 eV energy loss), which is typically the case for core loss signals of heavier
elements such as Au. For these signals, EDX is the preferred technique.
