Laboratory for Electron Microscopy

Nanoparticles and clusters

Nanoscaled particles exhibit a wide range of fundamentally and technologically interesting properties which are mainly determined by their size and structure. High-resolution transmission electron microscopy (HRTEM) using the aberration-corrected Titan3 80-300 microscope allows detailed studies of cluster morphologies. The images below shows HRTEM images of Pt clusters with fcc cuboctahedral structure in the [110]- (image (a) below) and in the [100]-zone axis (image (b) below).

The images below demonstrate the strong influence of different substrates on the formation of Pt-clusters after electron-beam evaporation. Deposition on highly-oriented pyrolytic graphite (a) facilitates the formation of crystalline clusters. Deposition on amorphous carbon films (b) and self-assembled monolayer films (c) (fabricated by A. Beyer, University Bielefeld) leads the less-ordered structures which can be in extreme cases disordered agglomerates of Pt-atoms.

High-resolution transmission electron microscopy is also applied frequently to analyse the structural properties of novel nanoscaled particles. Fig. 2 displays a hollow gold sphere with a wall thickness of 2 to 3 nm (Fig. 2 a)) and a YVO4 – YF3 core-shell cluster (Fig. 2 b)), which were produced in the group of C. Feldmann (Institute of Inorganic Chemistry, University of Karlsruhe). The YVO4 – YF3 core-shell cluster has an external diameter of D=22 nm and a core diameter of 8.7 nm, which reveals the “epitaxial” relationship between the core and the shell structure of these clusters: YVO4[001]//YF3[100].

We also study the mean inner Coulomb potential of nanoscaled clusters which is determined by the volume-averaged electrostatic part of the crystal potential using off-axis electron holography. A strong increase of the mean inner potential is observed for Au clusters from 30 V for bulk gold up to 80 V for clusters with a radius of 0.5 nm (see poster and paper below for details). The increase of the mean inner Coulomb potential has serious consequences for the quantification of electron scattering data because the mean inner potential determines the amplitude of the electron wave scattered in forward direction. For small Au clusters the electron wave scattered in forward direction is significantly stronger compared to bulk gold.

Other application-relevant issues are the stability of the cluster-size distribution and the immobilization of clusters on a suitable substrate. Coarsening of the cluster-size distribution will typically occur by Ostwald ripening and Brownian motion resulting in cluster fusion. Coarsening is technologically important, e.g. with respect to the efficiency of catalysts which requires nanoparticles within a small size interval. Using transmission electron microscopy, room-temperature coarsening of mass-selected Aun (n = 4, 6, 13 and 20) clusters produced in the group of M.M. Kappes (Institute for Physical Chemistry) was investigated. The clusters are deposited on amorphous carbon substrates, which leads to strong bonding of small clusters on the substrate and suppression of Brownian motion. Despite cluster immobilization, a significant increase of the average cluster radius with time is observed even at room temperature due to surface Ostwald ripening which is demonstrated in the diagrams below.

The figure above shows experimentally determined average radii of Au clusters as a function of time for Au4 (green), Au6 (blue), Au13 (brown), Au20 with a density of 2.12x1017 m-2 (red), Au20 with a density of 1.06x1017 m-2 (pink) and a paucidispersed initial clusters-size distribution of Aum clusters (10 ≤ m ≤ 20) (black). The solid lines of the corresponding color are the fit curves assuming diffusion-limited kinetics of surface OR.

Pt clusters are widely used as catalysts in the oil, chemical and automotive industry. The catalytic activity depends crucially on the cluster size and degrades if the cluster sizes increase. This motivates our studies of the coarsening mechanisms of Pt clusters deposited on amorphous carbon. The average Pt cluster radius increases with annealing time t and temperature T (see diagram below). The mass-transport diffusion coefficient Ds at the corresponding T can be calculated for t >2 h, when Pt clusters coarsen by surface Ostwald ripening (for details see the poster). An Arrhenius-type dependence of Ds(T) results in an activation energy for the surface diffusion of Pt atoms of Ed=0.85±0.09 eV/atom.



Selected conference poster presentations:

The structure of supercrystals made by self-assembled nanoscaled Ag2S hollow spheres and Ag2S nanodiscs (pdf)

TEM investigations of the sintering behavior of noble metal nanoparticles (pdf)

Coarsening of Pt clusters on amorphous carbon substrates (pdf)


Ausgewählte Veröffentlichungen:

[1]M. Wanner, D. Gerthsen, S.-S. Jester, B. Sarkar, B. Schwederski,
Treatment of citrate-capped Au colloids with NaCl: a TEM, EAS und EPR study of the accompanying changes,
Colloid and Polym. Sci. 283, 783 (2005)

[2] R. Werner, M. Wanner, G. Schneider, D. Gerthsen,
Island formation and dynamics of gold clusters on amorphous carbon films,
Phys. Rev. B 72, 045426 (2005)

[3] M. Dubiel, X. Yang, R. Schneider, H. Hofmeister, K.-D. Schicke,
Structure and properties of nanoparticle-glass composites
Phys. Chem. Glasses 46, 2, 148-152 (2005)

[4] M. Dubiel, X. Yang, R. Schneider, H. Hofmeister, K.-D. Schicke,
Structure of silver nanoparticles in silicate glasses and of nanoparticle-glass interfaces,
Phys. Chem. Glasses 46, 4, 389-393 (2005)

[5] M. Wanner, R. Werner, D. Gerthsen,
Dynamics of gold clusters on amorphous carbon films induced by annealing in a transmission electron microscope,
Surface Science 600, 632 (2006)

[6] C. Zimmermann, C. Feldmann, M. Wanner, D. Gerthsen,
Nanoscale gold hollow spheres via microemulsion approach,
Small 3, 1347 (2007)

[7] Radian Popescu, Erich Müller, Matthias Wanner, Dagmar Gerthsen, Marco Schowalter,
Andreas Rosenauer, Artur Bottcher, Daniel Loffler, Patrick Weis,
Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering,
Phys. Rev. B 76, 235411 (2007)

[8] M. Dubiel, R. Schneider, H. Hofmeister, K.-D. Schicke, J.C. Pivin,
Formation of argentic clusters and small Ag nanoparticles in soda-lime silicate glass,
Eur. Phys. J. D 43, 291-294 (2007)

[9] E. Prestat, R. Popescu, H. Blank, R. Schneider, D. Gerthsen,
Coarsening of Pt nanoparticles on amorphous carbon film,
Surf. Sci. 609, 195 (2013)

[10] G. Mayer, M. Fonin, U. Rüdiger, R. Schneider, D. Gerthsen, N. Janßen, R. Bratschitsch
Structural and optical properties of high-quality ZnO nanocrystals embedded in SiO2 fabricated by rf-sputtering
Nanotechnology 20, 075601 (2009)

[11] R. Popescu, R. Schneider, D. Gerthsen, A. Böttcher, D. Löffler, P.Weiss, M. Kappes
Coarsening of mass-selected Au clusters on amorphous carbon at room temperature
Surf. Sci. 603, 3119 (2009)

[12] P. Leidinger, R. Popescu, D. Gerthsen, C. Feldmann
Nanoscale La(OH)3 hollow spheres and fine-tuning of its outer diameter and cavity size
Small 6, 1886 (2010)

[13] G. Kiliani, R. Schneider, D. Litvinov, D. Gerthsen, M. Fonin, U. Rüdiger, A. Leitenstorfer, R. Bratschitsch
Ultraviolet photoluminescence of ZnO quantum dots sputtered at room-temperature
Optics Express 19, 1641 (2011)

[14] Li Shang, R.M. Dörlich, S. Brandholt, R. Schneider, V. Trouillet, M. Bruns, D. Gerthsen, G.U. Nienhaus
Facile preparation of water-soluble fluorescent gold nanoclusters for cellular imaging applications
Nanoscale 3, 2009 (2011)

[15] P. Leidinger, R. Popescu, D. Gerthsen, H. Lünsdorf, C. Feldmann
Nanoscale copper sulfide hollow spheres with “phase-engineered” composition: covellite (CuS), digenite (Cu1.8S), chalcocite (Cu2S)
Nanoscale 3, 2544 (2011)

[16] C. Zurmühl, R. Popescu, D. Gerthsen, C. Feldmann
Microemulsion-based synthesis of nanoscale TiO2 hollow spheres
Solid State Sciences 13, 1505 (2011)

[17] Li Shang, A. Naghmeh, F. Stockmar, W. Send, V. Trouillet, M. Bruns, D. Gerthsen, G. U. Nienhaus
One-pot synthesis of near-infrared emitting, dihydrolipoic acid capped gold clusters for cellular fluorescence imaging
Small 7, 2614 (2011)

[18] P. Leidinger, N. Dingenouts, R. Popescu, D. Gerthsen, C. Feldmann
ZnO Nanocontainers: Structural study and controlled release
J. Mater. Chem. 22, 14551 (2012)

[19] Li Shang, Linxiao Yang, F. Stockmar, R. Popescu, V. Trouillet, M. Bruns, D. Gerthsen, G.U. Nienhaus
Microwave-assisted rapid synthesis of fluorescent gold nanoclusters for imaging Hg2+ in living cells
Nanoscale 4, 4155 (2012)

[20] C. Kind, R. Popescu, R. Schneider, E. Müller, D. Gerthsen,  C. Feldmann
Phase-engineering of advanced bimetallic In–Cu/Ag/Au nanostructures via tailored Microemulsion-based Reaction
RSC Adv. 25, 9273 (2012)

[21] F. Gyger, P. Bockstaller, D. Gerthsen, Claus Feldmann
Ammonia-in-oil-microemulsions and their application
Angew. Chem. Internat. Ed. 52, 12443 (2013)

[22] P. Leidinger, R. Popescu, D. Gerthsen, C. Feldmann
Nanoscale Ag2S hollow spheres and Ag2S nanodiscs assembled to 3D nanocrystal superlattices
Chem. Mater. 25, 4173 (2013)

[23] H. Dong, R. Popescu, D. Gerthsen, C. Feldmann
Shape-stabilized Bi2Te3-capped Tellurium nanorods
Z. Anorg. Allg. Chem. 639, 2406 (2013)

[24] H. Dong, T. Schnabel, R. Popescu, D. Gerthsen, E. Ahlswede, C. Feldmann,
Se@CuSe Core@Shell nanoparticles: colloidally stable Se-precursor for thin-film manufacturing of CIS solar cells
J. Colloid Interface Sci. 415, 103 (2014)