Shinjiro Takano, Yuya Hamasaki, and Tatsuya Tsukuda, The University of Tokyo, Japan, have developed a gentler synthesis method to trap gold clusters at their earliest growth stages and determined their atomic structures using X-ray crystallography. They slowed down the reduction of gold ions. This allowed them capture gold clusters as they grew.

Key features of the team’s synthesis include using nearly equivalent amounts of thiolate and a much smaller amount of the mild reducing agent t-BuNH₂BH₃ relative to the Au(I) precursors. Using trimethylsilylmethanethiol (TMSCS-H) as a protecting ligand enabled them to crystallize clusters, including Au₁₅(SCTMS)₁₃, Au₁₈(SCTMS)₁₄, Au₂₂(SCTMS)₁₈, Au₂₃(SCTMS)₁₇, Au₂₅(SCTMS)₁₈, Au₃₃(SCTMS)₂₅, and Au₃₄(SCTMS)₂₆.

The team found that instead of the expected spherical 25-atom clusters, the gold atoms assembled into unusual triangular and tetrahedral building blocks that grew into ultrathin, pencil-shaped “quantum needles” with strong near-infrared light absorption. The “quantum needles” are so thin (about 1.8 nm thick) they’re only as wide as three gold atoms across. When excited by near-infrared light, they release energy at two wavelengths—one through fluorescence and the other through phosphorescence: For quantum wires made of 33 gold atoms, the peaks appeared at 840 and 1000 nm, while for those made of 34 gold atoms, they were at 830 and 970 nm.

According to the researchers, these insights clarify how gold clusters form at the atomic level and open new routes to design nanoscale materials for solar energy and biomedical applications.

• X-ray Crystallographic Visualization of a Nucleation and Anisotropic Growth in Thiolate-Protected Gold Clusters: Toward Targeted Synthesis of Gold Quantum Needles
  Shinjiro Takano, Yuya Hamasaki, Tatsuya Tsukuda
  J. Am. Chem. Soc. 2025
  https://doi.org/10.1021/jacs.5c11089