When most people think of gold, they picture jewelry or treasure. However, at the atomic scale, gold behaves in ways that are far more valuable to science than to fashion. 

Researchers from the University of Tokyo have managed to capture the very first steps in the growth of gold nanoclusters, particles made of just a few dozen atoms. This is something that scientists have been chasing for years.

Moreover, in the process, they stumbled across a completely new shape—long, needle-like clusters that they named gold quantum needles. 

According to the researchers, these needles interact strongly with light in the near-infrared range, and they could one day sharpen medical imaging and make devices that turn light into energy more efficiently.

The black box of nanocluster growth

An illustration of quantum needles. Source: Takano et al, 2025

Gold nanoclusters, which contain fewer than 100 atoms, are usually created by giving gold ions extra electrons while surrounding molecules called ligands keep them stable. The big problem is control. These clusters often grow unpredictably, making it nearly impossible to produce specific shapes or sizes at will. 

Researchers know that structure determines how nanoclusters behave, but the moment when structure first begins to form has remained hidden.

“Over the past years, much effort has been devoted to understanding the correlation between the structure and physicochemical properties of the nanoclusters. However, the formation process is regarded as a black box,” Tatsuya Tsukuda, one of the study authors and a chemistry professor at the University of Tokyo, said.

To solve this mystery, the researchers used an unconventional trick. They deliberately slowed the growth process by changing the normal reaction conditions. This gave them a rare chance to freeze clusters during their earliest moments, before they morphed into more stable forms.

They then analyzed the captured clusters using single-crystal X-ray diffraction, a technique that reveals exactly where each atom sits. The results were unexpected. Instead of forming evenly in all directions, the clusters grew unevenly, faster in some directions than others. 

Even more surprising, the atoms arranged themselves into elongated shapes built from repeating units of three (trimers) and four (tetramers) gold atoms. As electrons inside these slender structures could only occupy fixed energy states, which is a hallmark of quantum behavior, the team named them quantum needles.

“The formation of needles with a base of a triangle of three gold atoms instead of a nearly spherical cluster is a serendipitous finding that was far beyond our imagination,” Tsukuda said.

Significance of the cluster formation process

By mapping the step-by-step growth of nanoclusters, the study authors have provided a rare glimpse into how atomic building blocks come together. This could lead to the development of new methods to craft clusters with desired shapes and properties, rather than leaving growth to chance. 

Moreover, the quantum needles themselves may find real-world uses thanks to their ability to interact with near-infrared light, a feature valuable for sharper biomedical imaging and for converting sunlight into usable energy.

However, producing these needles in large numbers and making changes to them for practical devices will take more work. The study authors now plan to fine-tune their synthetic methods and explore whether other unusual shapes can be made out of gold or even different metals. 

The study has been published in the Journal of the American Chemical Society.