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C4.6: Support Interactions and Thermal Stability of Size Selected Cluster Deposited onto Single Crystal Surfaces

Subproject Leader: Manfred Kappes

Institut für Pysikalische Chemie, KIT

 

Contributing Scientists:

Present: Stefanie Klumpp, Matthias Vonderach, Rebecca Kelting, Dmitry Strelnikov, Tatjana Karpuschkin
Past: Oliver Ehrler, Ahmad Ayesh, Thomas Rapps, Anne Lechtken

 

Structures of Gold cluster anions Au11--Au20- as determined by TIED.
Structures of Gold cluster anions Au11--Au20- as determined by TIED.
CO binding energies to AunAum+ (m+n = 5 top, m+n = 6 bottom)
CO binding energies to AunAum+ (m+n = 5 top, m+n = 6 bottom)
Comparison of calculated and experimental collision  cross sections of Boron cluster cations.
Comparison of calculated and experimental collision cross sections of Boron cluster cations.
Structures of small tin cluster anions and cations together with calculated and experimental collision cross sections.
Structures of small tin cluster anions and cations together with calculated and experimental collision cross sections.
Structures of selected Sn18- - Sn25
Structures of selected Sn18- - Sn25
AFM images of C58/HOPG films created using different deposition conditions
AFM images of C58/HOPG films created using different deposition conditions: A) variation of surface temperature, B) variation of incident kinetic energy and ...
C) prepatterning of surfaces
C) prepatterning of surfaces (left: amorphous carbon defects in HOPG; right C58 islands generated by deposition onto such HOPG defects).
Exploring reaction of non-IPR fullerene thin films with atomic hydrogen (I):  (T,Mass) sublimation maps taken before (a) and after (b) hydrogenation of a thick C58 films.
Exploring reaction of non-IPR fullerene thin films with atomic hydrogen (I): (T,Mass) sublimation maps taken before (a) and after (b) hydrogenation of a thick C58 films.
AFM images
Exploring reaction of non-IPR fullerene thin films with atomic hydrogen (I): AFM images taken before (left panel) and after the hydrogenation of a thin C58 film (right panel).

Support Interaction and Thermal Stability of Size Selected Cluster Deposited onto Single Crystal Surfaces

Top down nanofabrication in its quest for smaller structures eventually runs into the problem of granularity. Chemists call these grains clusters. Harnessing such clusters and their properties is one of the greatest challenges for atomically precise nanotechnology. Towards this end it is necessary to understand structure and quantum size effects for clusters - both in their isolated state as well as under realistic, surface trapped conditions. We pursue these two goals in this subproject.

A: Gas Phase Structural Probes of Clusters

We have studied a number of gas-phase, elemental cluster systems, with the aim of determining the evolution of cluster structure with number of constituent atoms. The main experimental techniques we use are gas phase ion mobility spectrometry (IMS) and trapped ion electron diffraction (TIED). In addition, ion chemistry of metal cluster ions is probed in an ion cyclotron resonance mass spectrometer (FT-ICR). Below we provide a few recent examples of our work.

Gold cluster anions Aun-(n≤20) show a wide diversity of structural phases ranging from planar (n=4-12) through three-dimensional “flat-stacked” (n = 12-15), hollow (m=16-18), and fcc-like (n=19, 20) structures. Interestingly, most clusters in this size range appear to comprise a single dominant isomer. In a few cases, the experimental data could only be fit by assuming a superposition of multiple coexisting isomeric forms. Typically the experimentally best fitting structure is either the DFT global minimum or an energetic close-lying isomer. However, comparison to experiment has uncovered systematic problems of DFT descriptions for certain cluster sizes, e.g. in the region of the 2D to 3D transition. At larger sizes (n=34), compact surface reconstructed structures were found.[1]

Small mixed gold-silver clusters AunAgm+ (n+m = 4-6) are prototypic clusters of a fully miscible alloy. Detailed adsorption kinetics were studied and the resulting absolute rate constants were analyzed to yield binding energies of carbon monoxide. A CO molecule always binds in an atop position on a single gold atom with a binding energy that generally decreases with increasing number of neighboring silver atoms. This can be understood by the increased charge density on the gold atoms due to their higher electronegativity (relative to silver) and the associated weakening the σ-donor bond of CO. In principle, "tunable" binding energies by way of varying the composition may be relevant to catalytic processes where specific reaction steps needs to be properly activated.[2]

We have determined the structures of boron cluster cations using ion mobility spectrometry and quantum chemical calculations: they are planar up to B15+, starting at B16+ we find cyclindrical structures (double rings comprised of connected B3-triangles). Icosahedral structures, prominent in bulk phase boron, are completely absent in boron clusters with less than 25 atoms. Instead, we see a tendency towards cylinders having two or more boron ring segments, i.e. towards single-walled boron nanotubes [3].

Tin clusters are promising candidates for cluster materials since their binding energies as function of cluster size quickly converge to the bulk value. We have determined the structures of positively and negatively charged tin clusters by IMS and TIED. Small clusters have (quasi) compact structures. Sn12-is an icosahedral-cage-structure. For larger clusters Sn13- - Sn25- prolate structures were found. Interestingly, these clusters are built from multiply-repeating subunits, which are either face connected or which form dimers/clusters of subunits (mainly tricapped trigonal prism and/or bicapped quadratic antiprism). This rather unusual growth mode can be rationalized by the high stability of the subunit clusters.[4]

B: Size Selected Clusters on Single Crystal Surfaces

Concerning clusters on surfaces, wehave focused on the fabrication of new monodisperse materials consisting of mass-selected non-IPR fullerene cages. The corresponding growth procedure is based on soft landing of the mass-selected cluster ions onto well-defined substrates (i.e. HOPG, Au(111), a-C, and others). The non-IPR carbon cluster building blocks, Cn, were created via electron impact ionization/fragmentation of larger IPR fullerenes (C60 and C70). The thin film deposits were prepared under ultra-high vacuum conditions via selecting the desired ions, Cn+ (46 < n < 70) and collecting them on inert surfaces at low incident kinetic energies (< 6 eV). Non-IPR fullerene cages may be thought of as  functionalized (ligand free) carbon clusters, i.e. the non-IPR sites located on the cage “surfaces” exhibit enhanced reactivity allowing e.g., for covalent cage-cage interconnection. Consequently such systems are of potential interest for all-carbon nanotechnology: the sub-nm, building blocks are quite stable and can be easily positioned, manipulated and used as a patterning medium on surfaces. The patterns created by soft-landing can be readily tuned by controlling deposition conditions or by depositing the ions onto intentionally (pre-)patterned surfaces [5].

The thermal stability of the films is governed by the quasi-covalent intercage bonds formed between non-IPR sites on adjacent cages, i.e. in the fully coordinated solid, non-IPR fullerenes can interlink to form  -Cn-Cn-Cn- oligomers in 2- and 3-dimensions. Thermal desorption studies, TDS, reveal that the mean binding energy of the building blocks depends on the size of the individual cages. All non-IPR materials exhibit considerably higher thermal stability than do the corresponding parent IPR solids, C60 and C70[6].

The valence bands of the Cn solids reveal a clear dependence of the DOS on the size of the constituting cages. All non-IPR Cn solids exhibit lower surface ionisation potentials, Ip, than do the IPR films. Ip varies with n in the range from 6.4 eV up to 6.9 eV. All Cn materials appear to be wide band semiconductors with the HOMO-LUMO gap ranging from 0.7 to 1.9 eV.

DFT calculations have predicted that it should be possible to form “Stone-Walls transformed” non-IPR C60 cages (at elevated temperatures ~ 4400 K). These new stable C60 isomers should exhibit two non-IPR sites. We have been able to generate such cage isomers by electron-impact induced fragmentation of C70. By soft-landing these (and selectively subliming the more volatile C60(Ih) component of the deposited isomer mixture), we have fabricated a new monodisperse material consisting of the C60(C2v) isomers exclusively. This material shows unique electronic properties [7].

The hydrogenation/deuteration of non-IPR materials by reaction with atomic hydrogen has been investigated as a simple indicator of reactivity (and of reactivity differences to IPR materials). Progressing formation of CnHx hydrides reduces the mean binding energy of -Cn-Cn- oligomers. Finally, oligomers decompose entirely into hydrogenated Cn cages. The transformation of dendritic islands into compact smooth rimmed islands is an indicator of the associated cleavage of covalent intercage bonds in the oligomers – to yield hydrogenated cages with much weaker van der Waals interactions.

References

[1]

Structure determination of gold clusters by trapped ion electron diffraction: Au14--Au19-. A. Lechtken, C. Neiss, M. M. Kappes, D. Schooss, Physical Chemistry Chemical Physics 2009, 11, 4344, http://dx.doi.org/10.1039/b821036e

[2]

Binding energy and preferred adsorption sites of CO on gold and silver-gold cluster cations: Adsorption kinetics and quantum chemical calculations, M. Neumaier, F. Weigend, O. Hampe, M. M. Kappes, Faraday Discussions 2008, 138, 393, http://dx.doi.org/10.1039/b705043g

[3]

Boron cluster cations: Transition from planar to cylindrical structures, E. Oger, N. R. M. Crawford, R. Kelting, P. Weis, M. M. Kappes, R. Ahlrichs, 16 2007, 46, 8503, http://dx.doi.org/10.1002/anie.200701915

[4]

Tin cluster anions (Snn-, n=18, 20, 23, and 25) comprise dimers of stable subunits, A. Lechtken, N. Drebov, R. Ahlrichs, M. M. Kappes, D. Schooss, Journal of Chemical Physics 2010, 132, 211102 http://dx.doi.org/10.1063/1.3442411

[5]

Morphology of C-n thin films (50 <= n < 60) on graphite: Inference of energy dissipation during hyperthermal deposition, S. S. Jester, D. Loffler, P. Weis, A. Bottcher, M. M. Kappes, Surface Science 2009, 603, 1863, http://dx.doi.org/10.1016/j.susc.2008.10.051

[6]

 Properties of non-IPR fullerene films versus size of the building blocks, D. Loffler, S. Ulas, S. S. Jester, P. Weis, A. Bottcher, M. M. Kappes, Physical Chemistry Chemical Physics 2010, 12, 10671, http://dx.doi.org/10.1039/c0cp00137f

[7]

Non-IPR C-60 solids, D. Loffler, N. Bajales, M. Cudaj, P. Weis, S. Lebedkin, A. Bihlmeier, D. P. Tew, W. Klopper, A. Bottcher, M. M. Kappes, Journal of Chemical Physics 2009, 130, http://dx.doi.org/10.1063/1.3120287


 

List of Publications 2006-2011 as PDF

Subproject Report 2006-2010 as PDF