Diffraction Measurements of Metal Cluster Symmetry

The study of small metal clusters has made extensive contributions to understanding the size-dependent, many-body character of nanoscale physics and chemistry. Important examples which have increased our appreciation of the different forms in which size dependence is manifest include measurements and calculations of metal cluster melting, the transition of planar to three-dimensional structures and the reactivity of gold cluster nanocatalysts. Measurements conducted in our laboratory investigate the development of cluster structures with size range to develop an understanding of how these structures evolve through intermediate sizes to achieve “magic number” structures composed of closed electronic or atomic shells. The structural symmetries of cluster ions stored within a quadrupole ion trap are probed by electron diffraction as a function of cluster size and temperature [1]. The experimental configuration enables the accumulation of size selected clusters, collisional relaxation of the vibrational energy and adequate exposure time to collect electron diffraction data from ^~104 clusters. It is precisely the ability to isolate a single cluster size having a well defined temperature which provides for a controlled investigation of quantum size effects.

Figure 1

Figure 3

The diffraction instrument design shown in Fig. 1 incorporates a rail structure which allows the entire beamline to be extracted from the UHV chamber. The diffraction beamline shown in Fig. 2 is mounted within the UHV chamber and includes an rf trap, Faraday cup and microchannel plate detector configured to maintain a cylindrical symmetry around the electron beam axis. A CCD camera at the UHV window records the detector phosphor screen image of the electron diffraction pattern.

Figure 2

The sputter ion aggregation source shown in Fig. 3 produces metal cluster in the size range 10<n<100, of interest for current experiments. A quadrupole bender directs the cluster beam to a time of flight mass spectrometer to optimize cluster production in a particular size range after which the bender voltages are changed to direct the ion beam to the trap endcap electrode for ion loading.

(CsI)_nCs⁺ Diffraction

Diffraction experiments [2] on (CsI)_nCs⁺ clusters over the size range n=30-39 revealed that all cluster sizes except n=32 display patterns described by the NaCl rock salt structure of simple cubic symmetry. Data for n=32 indicate that contributions to diffraction are dominated by the CsCl structural isomer having bcc symmetry. This cluster size is unique in that it can form a closed shell rhombic dodecahedron corresponding to the CsI bulk bcc structure. The structural transition observed in these experiments was induced by the change of a single molecule, demonstrating both the enhanced stability of closed shell structures and the importance of measurements on a single cluster size. It will be important to push this technique to a level to enable diffraction measurements on small cluster sizes of 10-30 atoms.

Figure 4

Measurements on (CsI)₁₃ Cs⁺ clusters shown in Fig. 4 indicate the feasibility of extending diffraction to smaller cluster sizes. This figure shows a CCD image of the electron diffraction ring pattern. (a) A plot of the integrated ring intensity as a function of momentum transfer (s) proportional to scattering angle. (b) The molecular diffraction intensity sM(s) extracted from the ring pattern is compared with a calculated intensity assuming a simple cubic (3x3x3) structure.

Metal Cluster Diffraction: Ag_n⁺ n=36-55

The study of small metal clusters has made extensive contributions to understanding the size dependent, many-body character of nanoscale physics and chemistry. Important examples which have increased our appreciation of the different forms in which size dependence is manifest include measurements and calculations of metal cluster melting, the transition of planar to three dimensional structures, and the reactivity of gold cluster nanocatalysts. Trapped ion electron diffraction measurements on silver cluster cations present the first measurements of the structure of mass selected metal clusters by trapped ion electron diffraction. Diffraction techniques have been shown to be particularly sensitive to the measurement of size dependent changes in structural symmetry for small clusters. Measurements recently completed investigate the development of silver cluster structures over the size range 36 to 55 atoms to develop an understanding of how cluster structures evolve through intermediate sizes to achieve “magic number” structures composed of closed electronic or atomic shells. These measurements [3] have discovered an evolution from short range order among nearest neighbors having fivefold symmetry to a global order having icosahedral symmetry at n=55. The local fivefold symmetry does not result from the decoration of an inner icosahedral core of 13 atoms having Ih symmetry. Not a single isomer calculated for cluster sizes below n=55, comprising a total of ~40 DFT optimized cluster structures, contains an icosahedral core. The local order in these calculated structures becomes apparent only after symmetry analysis of the cluster structure.

Current Diffraction Measurements

Diffraction measurements of gold cluster anions were performed throughout the cluster size range 11<n<23. This series of anion cluster sizes has been very actively studied, both theoretically and experimentally. Theory predicts a 2D to 3D transition for surprisingly large size (n=13). Gas phase mobility measurements observed a change in cross section which correlated with a calculated structural change from 2D to 3D at n=12-13. Catalytic reactions of adsorbed O₂ and CO also has been identified to take place on gold clusters residing on MgO surfaces. CO₂ formation at low temperatures (140 K) was found to require the back donation of negative charge. Experiments underway and those planned will investigate the structures, symmetry and catalytic processes on gold anions Au_n^- in this size range.