Letter pubs.acs.org/NanoLett
Experimental Evidence for Fluctuating, Chiral-Type Au55 Clusters by Direct Atomic Imaging Z.W. Wang and R. E. Palmer* Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, United Kingdom S Supporting Information *
ABSTRACT: We report the atomic-scale structures and fluctuating dynamical behavior of size-selected Au55 clusters obtained by aberrationcorrected scanning transmission electron microscopy (STEM) coupled with systematic STEM simulations. No high-symmetry structures (facecentered cubic polyhedron, icosahedron, or decahedron) were observed in our statistical investigation. We find Au55 clusters that are characteristic of the theoretically predicted chiral structure and similar sister isomers (which together we define as the chiral structural zone). The chiral structural zone was found to arise repeatedly in the time-lapse sequences of images we measured, though other amorphous-like structures are also frequently observed. The approach demonstrated here can be applied to identify specific low-symmetry atomic structures in other small clusters and distinguish them unambiguously from high-symmetry isomers. KEYWORDS: Size-selected, clusters, gold, chiral, aberration-corrected, STEM
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to systematic image simulations. No high-symmetry structures (Ih, Dh or fcc) were observed in an investigation of 27 Au55 clusters. Chiral clusters have obvious applications in enantioselective catalysis and perhaps also biomolecular binding.18,19 The Au55 clusters were generated in a magnetron sputtering, gas condensation cluster source and size selected with a lateral time-of-flight mass filter with a nominal mass resolution of 5% (i.e., they contain 55 ± 1 atoms).20−22 The clusters were deposited onto Cu grids covered with amorphous carbon films at soft-landing energies of either 60 or 110 eV. The STEM investigation was performed in a 200 kV JEOL 2100F instrument equipped with a CEOS spherical aberration probe corrector. The high-angle annular dark field (HAADF) images23−28 were acquired with inner and outer collection angles of 62 and 164 mrad and probe convergence angle 19 mrad. Each image was obtained within a few seconds, mostly with an electron dose of ∼6.9 × 103 electrons Å−2 as the beam scanned across a field of view of 13.3 × 13.3 nm. A time-lapse image sequence was recorded for each cluster, containing from a few to tens of frames. Figure 1 shows by way of example a sequential set of HAADF images of one Au55 cluster. The significant variation of the intensity and position of the atomic columns in the cluster can be seen clearly, arising from structural fluctuations and/or the rotational/rolling movement of the cluster during the acquisition of the image series. Fluctuations of the atomic structure have been observed before in small clusters,29−31 generally at an increasing rate with decrease of the cluster size.
ize-selected atomic clusters are one of the foundations of nanoscience.1−10 The unambiguous determination of their atomic structures has attracted wide interest and some progress has been achieved over the past decades. For gold clusters, density functional theoretical (DFT) calculations in combination with photoelectron spectroscopy, vibrational spectroscopy, or trapped-ion electron diffraction have been successfully applied to obtain the atomic structures of a series of gold nanoclusters in the gas phase, for example, Au7 presents a characteristic two-dimensional structure,11 Au20 a tetrahedral pyramid structure,11,12 and Au34 a possibly chiral structure with C3 point symmetry.13 Nevertheless, the atomic structures of most clusters remain unknown experimentally. Knowing the structures of the supported clusters is vital to the understanding of the mechanisms for heterogeneous catalytic reactions where the clusters function as nanocatalysts.14 Here we employ aberration-corrected scanning transmission electron microscopy (STEM) in conjunction with multiplescattering simulations to image directly Au55 clusters softlanded on amorphous carbon as a function of time with atomic resolution. The prominence of the Au55 cluster arises from Schmid’s proposal of a cuboctahedral magic number structure in its (weakly) ligated form,15 but photoemission spectroscopy measurements (indirectly) favor a low-symmetry or disordered structure for the gas phase cluster without ligands,16,17 while (diverse) theoretical predictions include an intriguing chiral geometry.18 Our systematic STEM investigation shows that Au55 clusters deposited on the carbon support fluctuate between a range of amorphous-like atomic geometries, among which the predicted chiral low symmetry geometry and its similar sister structures are observed and identified by reference © 2012 American Chemical Society
Received: June 24, 2012 Published: October 11, 2012 5510
dx.doi.org/10.1021/nl303429z | Nano Lett. 2012, 12, 5510−5514
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Figure 1. Individual frames from a sequence of HAADF-STEM images of a Au55 cluster. (a) Frame 4 (b) Frame 8, (c) Frame 11.
Figure 2. Atomic models and multislice simulations of Au55 clusters. (a−d), atomic models of cuboctahedral, Ino-decahedral, icosahedral, and chiral Au55 clusters, respectively. The insets on the right of (a−d) show the simulations along particular orientations shown on the left, specifically, the ⟨110⟩ axis, 5-fold axis, 5-fold axis, and θ=20°/α=90° (see Supporting Information), respectively.
Figure 3. HAADF images extracted from the sequences recorded for six different Au55 clusters. (a−c) Chiral-type structures; the insets are the simulations obtained using the chiral model for the orientations of (a) θ=150°/α=50°, (b) θ=130°/α=140°, and (c) θ=30°/α=90° followed by a rotation of 180° around the Z-axis. The arrows in (b) and (c) mark characteristic structural features discussed in the main text. (d−f) Amorphouslike structures that are not consistent with any of the models investigated (i.e., Ih, Dh, fcc, and the chiral).
The driving force for the fluctuations is likely to be a mixture of thermal energy and electron beam interaction with the clusters, even given the low electron dose used to record the images. The key concept is that these effects cause the cluster to explore different configurations in the multidimensional potential energy surface. The possibility that the substrate interaction decreases the energy barrier between the different isomers may also come into play. Since the clusters locate on the substrate in random orientations, one cannot simply determine the cluster atomic structure by considering, say, high-symmetry isomers aligned with the electron beam direction. Thus, we performed systematic multiple scattering image simulations32 (using the
QSTEM software package33) for three typical highly ordered structures, icosahedral, decahedral, and cuboctahedral atomic models,7,34 as well as a chiral model having a large (111) facet with predicted lowest energy,18 so as to obtain a “simulation atlas” which covers all possible orientations of these cluster geometries on the surface, as shown in the Supporting Information. Figure 2 shows these atomic models and some examples of images from the simulation atlas. The atlas allows us to identify the principal image patterns characteristic of the different models, which is crucial in making the structural assignments (on a frame-by-frame basis). For example, the projections of the “crystalline” cuboctahedral isomer generally show either a two-dimensional fringe pattern (when the cluster 5511
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Figure 4. Individual frames from a time-lapse series of HAADF-STEM images of a Au55 cluster. (a) Frame 6, (b) Frame 8, (c) Frame 9. The insets in (a−c) are the simulations obtained using the chiral model along the orientations (a) θ=10°/α=0°, (b) θ=110°/α=10°, and (c) θ=20°/α=90° followed by a rotation of 90° around the Z-axis anticlockwise. The angles marked in the experimental and simulated images in (a) are 165 and 153°, respectively.
is orientated “on-axis”, for example, along ⟨110⟩, as shown in the inset in Figure 2a) or a unidirectional fringe pattern (for “off-axis”). For the decahedron, there is a typical projection along its 5-fold axis, as shown in the inset in Figure 2b, where the images present five circularly symmetric twins, each of which contains crystalline {111} and {200} fringes. Under some orientations of this model, the simulated images exhibit single-twin-like features, such as at θ=0°/α=70° in Figure S1 (see Supporting Information). The icosahedral geometry presents a more diverse range of projections but a unique characteristic is the concentric or “double-circle” feature that arises if the orientation is close to the 5-fold symmetry axis, as shown in Figure 2c. Overall, the projections of these three models present rather simple and regular projection patterns, consistent with their high-symmetry geometric structures. By contrast, the simulation atlas of the amorphous-like chiral isomer shows rather complex structural characteristics. For example, in some cases, the chiral isomer exhibits a noncircular projection outline, as shown in the inset in Figure 2d, where a horseshoe-like cluster shape can be clearly seen. While circlelike features do arise in the chiral model as they do in Ih clusters, the former normally contain only one complete circle (see Supporting Information, Figure S1). The differences between the projection characteristics of different atomic models can thus allow us to identify and distinguish them unambiguously. We performed a systematic experimental investigation by recording a set of HAADF image sequences35 of 27 Au55 clusters. The careful structural analysis, which was performed for each of the image sequences by reference to the simulation atlas, does not show the existence of the three high-symmetry isomers, icosahedral (Ih), decahedral (Dh), and face-centered cubic (fcc). This is basically consistent with previous investigations of Au55 clusters in the gas phase by photoemission spectroscopy (low-symmetry or disordered structures are preferred).16,17 Thus, the substrate (amorphous carbon film) may not have a very significant effect on the cluster structures; one possible reason is that Au clusters soft-landed on the substrate present the pseudospherical shapes36 leading to a relatively weak interaction between the clusters and surfaces. We find that a certain proportion of the image sequences observed present principal features consistent with the simulated images of the chiral isomer in an appropriate orientation. Figure 3a−c shows several such example images. In Figure 3a, the cluster displays a circular-like feature, which matches well with the simulated image obtained using the chiral model along the orientation of θ=150°/α=50°. The image in Figure 3b presents several bright dots in a line, as marked by
the arrow, in addition to a circle feature, basically consistent with the simulation shown in the inset. The cluster in Figure 3c shows a mixture of circular-like feature, as marked with arrow 1, and crystalline-like lattice patterns, as marked by arrow 2. This striking combination is also found in the simulation atlas of the chiral model, as shown in the inset. Of course, there is some imperfection in the detail of the match between the experimental image and corresponding simulation, which we attribute mainly to the two factors. (i) The effect of the substrate. Although the interaction between the clusters and amorphous carbon film seems to be weak and local, it is obvious that the cluster−substrate interaction cannot be neglected completely. There is a possibility that the substrate will slightly modify the geometric shapes and electronic structures of the clusters, leading to an imperfect match with simulations of a free cluster. (ii) The effect of electron beam irradiation. Previous studies,29 including our own,30,31 show that electron beam irradiation causes atomic structural fluctuations and/or slight rotation or transition of the clusters during the collection of an image. Density functional theory (DFT) calculations37,38 for free Au55 clusters indicate there are other, amorphous-like structures with very similar atomic arrangements to the chiral model and only slight larger energy (with the energy difference even as small as ∼27 meV, comparable with the value of kT at room temperature). The small energy barriers between the chiral isomer and its similar sister structures may be overcome by a combination of thermal energy and electron beam irradiation, so structural transitions will occur during imaging. Thus, more precisely, the images we show in this study probably manifest a region of the potential energy surface corresponding to local-minimum around the chiral model, which we thus define as the chiral “structural zone”. Despite the fluctuations occurring during imaging that may lead to the superimposition of different isomers, the very similar atomic arrangements of this set of cluster structures explains why the major projection features of the chiral structure are preserved with clear contrast, as shown in Figure 3a−c. Although we frequently identify the chiral structural zone in the image sequences of different Au55 clusters, our statistical investigation shows that a majority of the images cannot be assigned to any of the models considered in this study. Some examples of these “uncertain” isomers are given in Figure 3d−f. The low-symmetry structural characteristics presented indicate the likelihood that other amorphous-like structures are presenting themselves in some Au55 clusters.. Notwithstanding the apparent existence of a range of amorphous-like isomers, careful analysis of the image sequences 5512
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approach demonstrated in this study may, of course, also be applied to study other small nanoclusters. It has been demonstrated that HAADF-STEM imaging, sometimes coupled with simulations, is a powerful method to obtain the three-dimensional structures of large clusters presenting crystalline or other high-symmetry forms.24 However, it is not clear that this approach carries over to very small clusters (
