Controlled nanostructuring of a gold film covered with alkanethiol SAM by low energy cluster implantation
Size matters in scandium doped copper clusters
Ostwald ripening - Movie
Doped AunX+ clustersIndividual metallic clusters on flat surfaces
Research is conducted on individual metallic clusters deposited on atomically flat surfaces.
We examine the influence of the substrate on shape, structure, and mobility of the deposited clusters and we also probe their electronic structure.
Several complementary techniques are used to obtain information on cluster shape, structure, and mobility, including
Scanning Tunneling Microscopy (STM), High Resolution Electron Microscopy (HREM), Auger Spectroscopy, and Reflection
High Energy Electron Diffraction (RHEED). The electronic properties of the clusters are examined with low temperature
Scanning Tunneling Spectroscopy (STS).
Current research topics
Quantum confinement in metallic nanoparticles
Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are ideally suited for detailed studies
of both the geometrical and electronic properties of individual nanoparticles on metallic substrates.
We have deposited preformed Co clusters under controlled ultra-high vacuum conditions onto clean Au(111) with low density,
well below complete coverage. Very small clusters (typically less than 20 atoms) exhibit a significant surface mobility with
subsequent aggregation, whereas larger clusters turn out to be immobile. From a systematic analysis of the cluster height
distribution, we infer that the approximately spherical clusters experience only a limited flattening. Their multilayered structure,
which sometimes is observed to exhibit hexagonal facets, points to truncated octahedral shapes. STS measurements on individual Co
clusters reveal the presence of various size- and shape-dependent maxima with large energy spacings of the order of 100 meV. These
findings provide direct evidence for the existence of strong electron confinement in the Co clusters stemming from delocalized Co valence electrons.
Alternatively Co islands were grown on Au(111) by atom deposition and self-organization in arrays of mono- and bilayer nanoscale islands
that often have an hexagonal shape. The process of self-organization is induced by the Au(111) herringbone reconstruction. By mapping the local
density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The Co surface
state electrons are observed to be strongly confined laterally within the Co islands, with their wave functions reflecting the island symmetry. The
experimental work was complemented with particle-in-a-box calculations, based on a newly developed variational method that can be applied to 2D boxes
of arbitrary polygonal shape. The experimental patterns fit nicely to the calculated wave functions for a box with a symmetry corresponding to
that of the measured island. The small size of the Co islands under study is observed to induce a strong discretization of the energy levels,
with very large energy separations between the eigenstates up to several 100 meV.
This work is done in collaboration with Dr. Koen Schouteden and Prof. Chris Van Haesendonck of the
Scanning Probe Microscopy Group
(Laboratory of Solid State Physics and Magnetism, K.U.Leuven).
- Low-Temperature Scanning Tunneling Microscopy and Spectroscopy Investigation of the Electronic Surface State of Self-Organized Cr Islands on Au(111), K. Schouteden, D.A. Muzychenko, P. Lievens, C. Van Haesendonck, J. Nanosci. Nanotechnol. 9 (2009) 6767
- A study of the electronic properties of Au nanowires and Au nanoislands on Au(111) surfaces, K. Schouteden, E. Lijnen, D.A. Muzychenko, A. Ceulemans, L.F. Chibotaru, P. Lievens, C. Van Haesendonck, Nanotechnology 20 (2009) 395401
- Low-temperature STM/STS investigation of nanostructures created by an STM tip on Au(111) surfaces, K. Schouteden, P. Lievens, C. Van Haesendonck, Appl. Phys. A 96 (2009) 409
- Fourier-transform scanning tunneling microscopy investigation of the energy versus wave vector dispersion of electrons at the Au(111) surface, K. Schouteden, P. Lievens, C. Van Haesendonck, Phys. Rev. B 79 (2009) 195409
- Morphology and electron confinement properties of Co clusters deposited on Au(111), K. Schouteden, A. Lando, E. Janssens, C. Van Haesendonck, P. Lievens, New J. Phys. 10 (2008) 083005
- Confinement of surface state electrons in self-organized Co islands on Au(111), K. Schouteden, E. Lijnen, E. Janssens, A. Ceulemans, L.F. Chibotaru, P. Lievens, C. Van Haesendonck, New J. Phys. 10 (2008) 043016
Clusters on an insulating anorganic layer
The study of the intrinsic properties of individual clusters with scanning tunneling spectroscopy (STS) currently is limited by the difficulty
to create well characterized and atomically flat insulating layers, which decouple the clusters from the substrate. Alumina, magnesium oxide,
silica, and sodium chloride (NaCl) are some of the proposed materials to support and electronically isolate clusters. Among those, crystalline
NaCl is very promising because it can be grown as atomically flat layers on various substrates stoichiometrically.
We have successfully grown crystalline NaCl bilayers on Au(111), well suited for studiess with atomically resolved scanning tunneling microscopy
(STM). STS measurements focussing on the gold surface state demonstrate the insulating characteristics of the NaCl.
This work is done in collaboration with Dr. Koen Schouteden and Prof. Chris Van Haesendonck of
the Scanning Probe Microscopy Group (Laboratory of Solid State Physics and Magnetism, K.U.Leuven).
Clusters deposited on self-assembled monolayers
Self-assembled monolayers (SAMs) of organic molecules provide a simple, flexible, and ordered system for tailoring
the interfacial properties of a variety of surfaces. Among these systems, widely referenced and used are self-assembled monolayers
of thiol molecules on gold surfaces. Using SAM layers on gold surfaces as substrates for cluster deposition is advantageous because
the SAM suppresses the interaction with the solid substrate, that way preserving the intrinsic structural and electronic properties of the deposited cluster.
With ambient-conditions scanning tunneling microscopy we showed, however, that preformed gold clusters deposited on a SAM of dodecanethiol
molecules on a Au(111) surface are located at the Au(111) – SAM interface. This implies that the clusters are implanted through the organic
monolayer, followed by molecular reorganization. This study shows that the clusters are immobilized below the monolayer. By comparison with
the formation of SAMs on a nanostructured gold surface, we have demonstrated that cluster deposition through the organic monolayer can be
used to control the patterning of a gold surface covered with an alkanethiol SAM. This study illustrates the importance of the delicate
balance between cluster kinetic energy and SAM cohesion energy for cluster deposition on top of an organic monolayer. Furthermore the
absence of cluster diffusion enables the control of the density and size of the nanostructures grown at the interface.
This work is done in collaboration with Dr. Koen Schouteden and Prof. Chris Van Haesendonck of the
Scanning Probe Microscopy Group (Laboratory of Solid State Physics and Magnetism, K.U.Leuven).
- Controlled nanostructuring of a gold film covered with alkanethiol SAM by low energy cluster implantation, A. Lando, K. Lauwaet, P. Lievens, PCCP 11 (2009) 1521
- Single-electron tunneling phenomena on preformed gold clusters deposited on dithiol self-assembled monolayers, K. Schouteden, N. Vandamme, E. Janssens, P. Lievens, C. Van Haesendonck, Surf. Sci. 602 (2008) 552-558
Single molecule fluorescence spectroscopy
We want to quantify the catalytic activity of certain metallic clusters using the single molecule fluorescence
spectroscopy technique, a technique that has been used recently to characterize the catalytic activity of
Au nanoparticles [Xu et al, Nature Materials 7, (2008) 992]. Changing parameters such as the size and composition
of the clusters will allow us clarify the influence of these parameters on the catalytic activity. Optimizing the
catalytic activity may imply increase of both sensitivity and selectivity.
This work is done in collaboration with Prof. Johan Hofkens of the Laboratory for Photochemistry &
Spectroscopy (K.U.Leuven) and Prof. Bert Sels of the Centre for Surface Chemistry and Catalysis (K.U.Leuven).
