Very recently, Chinese researchers found a crystallographic closest packing-6H left-handed helical arrangement, which represents a novel crystallographic closest packing since the report of the third one (4H arrangement ) that was found in bulk Ag in 1979 (the first two are well-known fcc and hcp arrangement).

Since the birth of metal nanoclusters, its structural attracts much attention and in particular its external structure of gold nanoclusters (the arrangement of the nanoclusters in crystal, i.e. crystallographic packing) has been in the spotlight.

In fact, only two crystallographic closest packings of gold nanoclusters have been reported thus far: One is the fcc crystallographic arrangement of Au₃₀(SR)₁₈ nanoclusters reported in early 2016 by JIN Rongchao's group from Department of Chemistry, Carnegie Mellon University; the other is the recently reported 4H crystallographic arrangement in Au₉₂ crystals from WU Zhikun's group, Institute of Solid State Physics.

So it still remains unclear whether there are any other crystallographic closest packing in gold nanoclusters and the arrangement patterns affect the gold nanocluster properties or not.

 Figure.1 Anatomy of the atomic structure of Au₆₀S₆(SCH₂Ph)₃₆ nanocluster. Note: Au and S atoms in different bonding types are shown in different colors, C and H atoms are omitted for clarity. (Image by GAN Zibao) 

To address these questions, WU Zhikun’s group in the Institute of Solid State Physics (ISSP), Hefei Institutes of Physical Science conducted a series of investigation.

They synthesized a novel gold nanocluster-Au₆₀S₆(SCH₂Ph)₃₆ via a thermal-induced ligand exchange reaction, and unveiled its crystal structure by single crystal X-ray crystallography (SCXC), revealing that Au₆₀S₆(SCH₂Ph)₃₆ consists of a fcc-like Au₂₀ inner core protected by a pair of giant Au₂₀S₃(SCH₂Ph)₁₈ staple motifs containing 6 tetrahedral-coordinate μ₄-S atoms not previously reported in Au-S interface (Figure 1).

They were interested to find that Au₆₀S₆(SCH₂Ph)₃₆ nanoclusters adopt the closest packing, and a very special stacking sequence of “ABCDEF” along the close-packed [001] direction in the single crystals (Figure. 2a).

 Figure.2 6HLH crystallographic arrangement of Au₆₀S₆(SCH₂Ph)₃₆ nanoclusters. (a) The stacking sequence along the [001] direction; (b) Front and (c) side views of the left-handed helical arrangement of Au₆₀S₆(SCH₂Ph)₃₆ nanoclusters; (d) the illustration of the left-handed helix. Note: To highlight the 6HLH arrangement, the Au atoms of the nanoclusters in each close-packed plane are labeled in different colors (Image by GAN Zibao). 

The arrangement of Au₆₀S₆(SCH₂Ph)₃₆ nanoclusters in single crystals along the [001] direction is reminiscent of the left-handed helix, here tentatively dubbed 6H left-handed helical arrangement (abbreviated as the 6HLH arrangement), as shown in Figure 4b-d (for clarity, a left-handed helical sequence is isolated from the crystal).

"Such a crystallographic arrangement is not only interesting but also exciting, as the third closest packing in crystals, named 4H, was found in 1979." said WU, the head of the research group .

Interestingly, the 6HLH arrangement gives rise to the obvious loss of solid photoluminescence of amorphous Au₆₀S₆(SCH₂Ph)₃₆(Figure 3), indicating the strong interaction among the uniquely arranged nanoclusters.

 Figure.3 The solid photoluminescence spectra of Au₆₀S₆(SCH₂Ph)₃₆ nanoclusters. (Image by GAN Zibao) 

This work provides a new strategy to the tuning of the solid photoluminescence of gold nanoclusters. As the reviewer commented, this work has important implications for the future search of novel crystallographic packing, and may also trigger more rigorous crystallographic research, including the study on the crystallographic packing-property correlation.

The study was detailed in Nature Communications entitled The fourth crystallographic closest packing unveiled in the gold nanocluster crystal, and the study is finanically supported by National Natural Science Foundation of China, Chinese Academy of Science, etc.