The optical properties of gold nanocrystals have a strong correlation with their shape. Although gold nanocrystals with a variety of shapes have been reported, it has been difficult to generate gold tetrahedra due to the necessary symmetry reduction relative to the cubic lattice of gold.

Younan Xia and co-workers at the Georgia Institute of Technology, Atlanta, GA, USA, have demonstrated a facile synthesis of gold nanocrystals with a tetrahedral shape through seeded growth. They used single-crystal, spherical Au nanocrystals (10 nm in size) as the seeds, together with HAuCl₄ as the precursor, ascorbic acid as the reductant, and a combination of cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB) as the capping agent or colloidal stabilizer.
These nanocrystals exhibited high uniformity in terms of both shape and size. Their sizes could be readily tuned in the range of 30–60 nm by simply varying the amount of precursor while fixing the amount of seeds. The researchers found that manipulation of the reaction kinetics by introducing the precursor with a syringe pump played a critical role in promoting a less symmetrical growth pattern on the spherical seed and, thus, the formation of truncated tetrahedron and tetrahedron.

• Seed-Mediated Synthesis of Gold Tetrahedra in High Purity and with Tunable, Well-Controlled Sizes,
  Yiqun Zheng, Wenying Liu, Tian Lv, Ming Luo, Hefei Hu, Ping Lu, Sang-Il Choi, Chao Zhang, Jing Tao, Yimei Zhu, Zhi-Yuan Li, Younan Xia,
  Chem. Asian J. 2014.
  DOI: 10.1002/asia.201402499