Ligand-protected clusters

One of the most commonly used cluster synthesis methods is the chemical colloidal method. In this method, the cluster is synthesized in the solution, but the bare clusters in the solution tend to be aggregated with each other. To protect the cluster from the aggregation, a stabilizing agent is used to form a ligand shell over the cluster metal core. This type of clusters is called ligand-protected cluster. ⁶⁰

Although metallic clusters were first synthesized quite a long time ago, the Au55(PPh3)12Cl6 first reported by Schmid in 1981 is important due to its unique 1.4nm size and a narrow size distribution.⁶¹ Due to its unique 1.4nm size, the Au55(PPh3)12Cl6 clusters have very special electronic properties that provide the potential for applications such as single electron switches and transistors in nanoscale electronic systems.⁹ The atomic structure of the Au55(PPh3)12Cl6 cluster metal core was firstly thought to be the cuboctahedron by Schmid, while some researchers

preferred an icosahedral structure. As we’ll see in Chapter 3, a hybrid structure with both fcc and icosahedral features was observed to be dominant in nearly half of the clusters by the high-resolution STEM study.⁶⁴

Au55(PPh3)12Cl6 clusters have phosphine ligands, which have a relative weak Au-P bond. Beside this type of ligands, thiol ligands are widely used to form thiol-protected clusters, sometimes called monolayer protected clusters.^65,66 Unlike the gold-phosphine bond, the gold-thiol bond is very strong and has significant effect on the surface of the cluster metal core. In 2006, Hakkinen et al theoretically predicted that the strong gold-thiol bond will form a gold-thiol cap on the surface of the pure gold core.⁶⁷ In 2007, this prediction was experimentally proved by Kornberg’s beautiful X-ray crystallography work on Au102 clusters.⁶⁸ Since then a series of studies also found this gold-thiol cap unit in the thiolate-protected gold clusters.^68–79 Although numerous theoretical works have been carried out on the atomic structures of the ligand protected clusters, the experimental side was not satisfactory for a long time, mainly due to the unsuccessful single crystallization of the clusters for the single crystal X-ray structure analysis. The breakthrough in this area was made by Roger Kornberg.⁶⁸

Kornberg et al. successfully crystalized (p-MBA) protected Au102 clusters and performed the single crystal X-ray diffraction structure analysis on them.⁶⁸ The determined atomic structure is shown in Figure 1.13. The atomic structure of the gold

core can be described as consisting of 4 parts: one 49-atom Marks-Decahedron, two 20-atom caps on the two poles of the five-fold axis in the Marks-Decahedron and one 13-atom band along the equator.⁶⁸ In this (p-MBA) protected Au102 cluster, only the 39 gold atoms in the core have pure Au-Au interaction. All the remaining 63 surface gold atoms have Au-S interactions, suggesting the strong influence of the thiol ligand on the cluster surface metallic atoms.⁶⁸ This Au-S staple was also found in other thiolate-protected clusters, which is now a common feature of this type of clusters.70,71,74,77,80,81

Figure 1.13 (a) The atomic structure of the Au102(p-MBA)44 cluster. Yellow is gold, cyan is sulfur, grey is carbon and oxygen is red. The red network is the electron density map. (b) The TEM image of Au102(p-MBA)44 clusters, showing a relative mono-dispersive size distribution. (c) The atomic structure of the Au core in the Au102(p-MBA)44 cluster. Yellow are the 49-atom Marks-Decahedron, green are the two 20-atom caps and blue is the 13-atom band.⁶⁸ The figure is from ref [68].

As well as the experimentally determined atomic structure, another interesting point of this study is the high stability of the Au102(p-MBA)44 clusters. This can be explained by the electronic shell structure with the jellium mode. In the Au cluster, each gold atom will provide one valence electron, so there are 102 valence electrons from the gold atoms. Then, the 44 thiol ligands will accept 44 electrons from the gold, so there are 58 electrons left in total.⁶⁸ As mentioned in section 1.1.2, the 58 electrons can fully fill the 1g shell, resulting in high stability. This is confirmed by a theoretical electronic structure calculation study.⁸² In this study, the HOMO-LUMO gaps of the bare Au102, Au102(SCH3)44, Au104(SCH3)46 and Au102(SCH3)42 clusters were calculated by the DFT method. The bare Au102 cluster has an 0.16eV HOMO-LUMO gap which is significantly smaller than the thiol protected Au102(p-MBA)44 cluster (~0.54eV), which indicates a significant influence of the ligand on the electronic structure. To further test the electronic structure’s effect on the cluster’s stability, the Au104(SCH3)46

cluster with 2 extra gold atoms and 2 extra thiol ligands was performed and the HOMO-LUMO gap is ~ 0.51eV. This also can be described as a 58 close electronic shell structure. Lastly, the Au102(p-MBA)42 cluster (which has the same number of gold atoms, but 2 less thiol ligands) has a zero HOMO-LUMO gap, which is due to the open electronic shell structure.⁸²

Following Kornberg’s work, more ligand protected clusters were successfully crystalized and their structures were determined by single crystal X-ray crystallography studies, such as Au18(SR)14, Au25-xAgx(SR)18, Au12Ag32(SR)30,

Ag44(SR)30, and Au133(SR)52. The single crystal X-ray crystallography is now routinely used to resolve the atomic structure of the protected metal clusters.^80,83–85 However, this method requires very high single crystal purity to achieve the atomic resolution.

The single crystal growth seems limited by the ligand type and the cluster size. Most successful crystallizations are found to have a ligand with a phenyl component.⁶⁰ Single crystallizations of the big size clusters also meet with difficulty. The biggest protected metal clusters are Au133(SR)52.⁸⁵ It is very difficult to form a high-quality single crystal (cannot get good X-ray diffraction signal) of the next promising cluster, Au144(SR)60,^86,87 suggesting either the structures of the Au144(SR)60 cluster is amorphous or the crystal has other size impurity.

As another powerful technique, high resolution electron microscopy does not require a high quality crystal or mono-dispersive size distribution, making it a more flexible structure characterization method. In the next section, we’ll introduce this technique applied to the nanoclusters in detail.