Determination
of Internal Structures of Heterogeneous
Nanocrystals Using Variable-Energy Photoemission Spectroscopy
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Abstract
This
article describes the determination of the internal structure
of heterogeneous nanoparticle systems including inverted core–shell
(CdS core and CdSe shell) and alloyed (CdSeS) quantum dots using depth-resolved,
variable-energy X-ray photoelectron spectroscopy (XPS). A unique feature
of this work is the combination of photoelectron spectroscopy performed
at lower X-ray energies (400–700 eV), to achieve surface sensitivity,
with bulk sensitive measurements at high photon energies (>2000
eV),
thereby providing detailed information about the whole nanoparticle
structure with a great accuracy. The use of high photon energies furthermore
allows us to investigate nanoparticles much larger than those studied
thus far. This capability is a consequence of the much-increased mean
free path of the photoelectron achieved at high excitation energies.
Our results show that the actual structures of the synthesized nanoparticles
are considerably different from the nominal, targeted structures,
which can be post facto rationalized in terms of the reactivity of
different constituents