X-Ray Photoelectron Spectroscopy of Metal & Metal-based Clusters: Au, Ag & Sn Cases.

Abstract

Clusters are particles which consist of a finite number (2 − 10 ^6−7 ) of atoms/molecules.
For the work presented in this thesis free clusters have been produced in order to study
their properties, free of interfering substrates, using photoelectron spectroscopy at the
MAX-lab synchrotron facility in Lund, Sweden. The clusters were produced using a
magnetron-based sputtering source and are considered large enough for their properties
to be adequately described using macroscopic descriptions.
Clusters of gold and silver have been produced and exposed to bromobenzene vapour
in order to examine the adsorption of bromobenzene to the clusters. The approximate
sizes of the clusters were determined using the liquid drop model and the fraction of their
surface area to which bromobenzene adsorbed was estimated via the relative intensities
of spectral components. Gold clusters were found to have a radius of 15 − 20 ¯ and a
coverage fraction of 0.58, while silver clusters were found to have a radius of 30−45 ¯ and
a coverage fraction of 0.76. The photoelectron experiments on free gold and silver clusters,
picking up bromobenzene molecules, have allowed us to shed additional light on the details
of bromobenzene adsorption; which is relevant to an important and sophisticated catalytic
process.
Studies of free tin and tin oxide clusters were also conducted and the chemical shift
of the 4d level between metallic tin and tin (IV) oxide was found to be ≈ 4.2 eV , not
the 2.0 eV previously reported 1 . Using the liquid drop model the size of metallic tin
clusters was found to be ≈ 3 − 5 nm. The tin oxide clusters have also been deposited
on a silicon substrate. The results of x-ray photoelectron studies conducted on these
deposited clusters of metallic tin, tin (II) oxide and tin (IV) oxide are consistent with
those conducted on free clusters. Size estimation via SEM images found deposited tin (II)
oxide and tin (IV) oxide clusters to have radii of approximately 10 nm and 10 − 20nm
respectively. The possibility to create nanoscale tin oxide of controlled composition is
important for the current activities in the field of large-gap transparent semiconductors.

Popular Abstract

Clusters of gold, silver and tin have been characterised via x-ray photoelectron spec-
troscopy (XPS) using x-rays produced at a synchrotron facility. Gold and silver clusters
were exposed to bromobenzene vapour to assess the extent to which adsorption of the
vapour occur. Clusters of tin were produced under the presence of several oxygen concen-
trations in order to determine the onset of tin clusters in their fully oxidised state. The
difference in energy required to eject a specific electron (called the binding energy) from
this oxidised state and from the metallic state was then found.
Clusters are particles which consist of a finite number of atoms/molecules and have prop-
erties between those of atoms and bulk materials. For the work presented in this thesis
free clusters have been produced in order to study their properties, free of interfering sub-
strates, using XPS at the MAX-lab synchrotron facility in Lund, Sweden. Clusters have
many applications in technology, both realised and potential, which may depend on the
unique properties of clusters or upon their much greater surface area to volume ratio with
regards to the bulk material. The high surface area to volume ratio of clusters is of inter-
est in catalysis, as they provide many sites on the surface where reactions can occur while
using little of the required material.
Adsorption of bromobenzene to gold and silver clusters has been studied because it is
a volatile chemical, released during the incineration of waste circuit boards, and there has
been a political movement to develop catalytic processes which would mitigate this hazard.
The process under discussion could involve gold or silver catalysts, for which adsorption
would be a first step. In this work adsorbed molecules were found to cover 60-80% of the
surface of the cluster.
Tin oxides are a topic of intense study in nanoscience, with applications in gas sensors
& transparent wide gap semiconductors. There are two stable forms, namely tin (II) oxide
and tin (IV) oxide, which are often present together in samples. Their different electronic
structures should make them readily identifiable by XPS. This has not been the case how-
ever and for tin (IV) oxide a change in binding energy of 2.0 eV from the metallic state has
previously been quoted: very different to the 4.0 eV change observed for similar electrons
in germanium (IV) oxide. The discrepancy has not been clearly explained and studies of
free tin and tin oxide clusters were conducted to clarify the change in binding energy be-
tween metallic tin and tin (IV) oxide. The results of this work ascertain a value of 4.2 eV .
Further work, following from the results of this thesis, could involve depositing
gold/silver clusters with adsorbed bromobenzene and exposing them to other reactants
in the cross-coupling reaction. The products of the reaction could then be controlled us-
ing a mass-spectrometer to determine whether the desired product has emerged. Further
experiments with tin could involve depositing tin clusters on transparent substrates and
measuring their absorption spectrum in the infrared, visible and ultraviolet regions. This
would permit one to establish the bandgap for the clusters: an important feature for po-
tential applications such as semiconductors.

• author: Wright, Charles
• supervisor: Stacey Ristinmaa Sörensen; Maxim Tchapylguine
• organization: Department of Physics; Synchrotron Radiation Research
• course: FYSM60 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Physics, Photoelectron, Spectroscopy, Cluster, Clusters, Gold, Silver, Tin, Synchrotron, X-Ray
• language: English
• id: 8888807
• date added to LUP: 2016-08-23 11:51:18
• date last changed: 2016-09-20 04:06:25